Take a dialer that places 10,000 answered calls. The same traffic, on the same route, at the same quoted rate of $0.010 per minute, can cost $70.50 or $140.00. Nothing about the calls changed. Only the billing increment did.
Billing increments are the rules a carrier uses to round call duration before applying the per-minute rate. “1/1” means a one-second minimum with one-second increments after that. “6/6” means six-second minimum and six-second increments. “60/60” means every call is rounded up to the next whole minute. “30/6” means a 30-second minimum, then six-second increments. For long calls the difference is noise. For short-call traffic it can double your bill.
The arithmetic below is illustrative. The rate and the call mix are invented to show the shape of the problem, not to represent any carrier’s pricing. The shape is the point.
Why the quoted rate is the wrong number to compare
The number that matters is the effective rate: what you pay divided by the minutes your calls actually lasted. Quoted rates only compare cleanly when the increments match.
A rate deck is a list of prices per destination. Increments often sit in a separate column, or in the commercial terms, or occasionally in a footnote you find after the first invoice. Two decks can quote the same destination at the same price and produce invoices that differ by 50% or more for the same traffic.
That’s why procurement teams that compare decks line by line on price alone tend to pick the wrong route for short-call use cases. The cheaper-looking deck is sometimes the more expensive one.
The worked example
Here’s an illustrative traffic profile for an outbound dialer. It’s 10,000 answered calls. The mix is deliberately short-heavy, which is common in dialer traffic because many answered calls end quickly: wrong person, quick decline, a voicemail greeting that the dialer drops on.
| Call length | Share of calls | Calls | Actual seconds |
|---|---|---|---|
| 7 seconds | 40% | 4,000 | 28,000 |
| 25 seconds | 30% | 3,000 | 75,000 |
| 70 seconds | 20% | 2,000 | 140,000 |
| 180 seconds | 10% | 1,000 | 180,000 |
| Total | 10,000 | 423,000 (7,050 min) |
Now apply each increment scheme at the same $0.010 per minute.
1/1. You pay for actual seconds. 7,050 minutes, $70.50.
6/6. Each call rounds up to the next multiple of six. The 7-second calls bill as 12, the 25-second calls as 30, the 70-second calls as 72, and the 180-second calls stay at 180. That’s 462,000 seconds, or 7,700 minutes. $77.00, about 9% above 1/1.
30/6. The 7-second and 25-second calls both bill as 30 seconds. The 70-second calls bill as 72. That’s 534,000 seconds, or 8,900 minutes. $89.00, about 26% above 1/1.
60/60. Every call under a minute bills as 60 seconds; the 70-second calls bill as 120. That’s 840,000 seconds, or 14,000 minutes. $140.00, nearly double.
| Scheme | Billed minutes | Cost at $0.010 | Overhead vs 1/1 |
|---|---|---|---|
| 1/1 | 7,050 | $70.50 | baseline |
| 6/6 | 7,700 | $77.00 | +9.2% |
| 30/6 | 8,900 | $89.00 | +26.2% |
| 60/60 | 14,000 | $140.00 | +98.6% |
The 60/60 line is the one that catches people. Almost none of the overhead comes from the long calls. It comes from the 4,000 seven-second calls, each billed at more than eight times its length.
The break-even rate: how to compare decks with different increments
The fair comparison is the rate at which a coarser increment costs the same as per-second billing for your traffic. Divide the 1/1 rate by the overhead multiplier.
For this call mix:
- A 6/6 route must be priced at or below about $0.00916 to match $0.010 on 1/1.
- A 30/6 route must be priced at or below about $0.00792.
- A 60/60 route must be priced at or below about $0.00504.
Read that last line again. For short-heavy traffic, a 60/60 route has to be roughly half the price of a per-second route just to break even. Carriers that sell 60/60 at a small discount to their 1/1 price are relying on buyers not doing this calculation.
The multiplier depends entirely on your call-length distribution. A conferencing platform with long average calls might see a 60/60 overhead of a few percent. A verification flow, where the typical call reads a code and hangs up, sits at the other extreme. Our post on voice OTP fallback explains why those calls are short by design.
Where the overhead hides in real traffic
The worked example uses four neat buckets. Real traffic has three features that make the overhead larger than a quick estimate suggests.
The short-call tail is fatter than you think
Most teams know their average call duration. Few look at the distribution. Averages flatter coarse increments, because a handful of long calls pulls the mean up while the short calls do the damage. Pull your CDRs and plot the histogram of billed duration in seconds. If there’s a spike under 15 seconds, increments matter more to you than rate.
False answer supervision inflates short calls
On some low-quality routes, the far end signals “answered” before a human picks up, sometimes while it’s still ringing or playing a recording. That’s false answer supervision. It turns unanswered attempts into billable calls, and every one of them is short. On a 60/60 route, each falsely answered call costs a full minute.
FAS shows up in your data as a pattern: unusually high answer rates, a cluster of calls lasting only a few seconds, and complaints from agents about “dead air” connections. The ASR, ACD and PDD guide shows how to read those metrics together.
Minimums and connection fees stack with increments
Some decks add a per-call connection charge or a minimum charge on top of the increment. A $0.001 connection fee sounds trivial. On 10,000 calls it’s $10, which in our example is 14% of the entire 1/1 bill. Always model the invoice, not the rate.
Mixed increments across one deck
Large decks rarely use a single increment everywhere. A carrier may bill most destinations 1/1 or 6/6 and apply 60/60 to specific countries, often where the upstream terminating operator bills that way. Mobile and fixed prefixes within one country can differ too; the mobile vs fixed termination guide covers why those prefixes are priced separately in the first place.
This creates a trap for blended analysis. If you compute one overhead multiplier across your whole traffic and apply it to a mixed deck, you’ll misstate the cost of exactly the destinations with the worst terms. The right approach is per destination:
- Group your CDRs by destination prefix, matching how the deck is structured.
- For each group, compute the overhead multiplier under that destination’s increment.
- Multiply by the quoted rate to get an effective rate per actual minute.
- Rank routes by effective rate, weighted by your volume in each destination.
This is spreadsheet work, not engineering. It usually takes an afternoon and changes at least one routing decision.
Auditing your invoice with re-rating
The strongest protection is to re-rate your own CDRs against the agreed terms and compare the total to the carrier’s invoice. Differences should be small and explainable.
Re-rating means taking each call record, applying the contracted increment and rate for its destination, and summing. You need the call’s start time, answer time, end time, destination number, and the route it went out on. Most switches and SBCs can export this.
Common causes of mismatch:
- Duration measurement differs. Your switch measures from 200 OK to BYE; the carrier may measure slightly differently. Small, consistent gaps are normal.
- Prefix matching differs. A number you treated as fixed may match a mobile prefix in the carrier’s deck, particularly after number porting.
- Deck changes applied mid-cycle. Rate notifications with short notice periods can change prices partway through a billing period.
- Increment applied incorrectly. Less common, but it happens, and it’s exactly what re-rating catches.
If the gap is consistently more than a percent or two, raise it with evidence. Carriers respond to CDR-level disputes much faster than to “our bill looks high.”
When coarse increments make sense
Coarse billing isn’t automatically a bad deal. It’s a price structure, and there are cases where it’s the right one to accept.
If your traffic is long-duration (conferencing dial-outs, call-center inbound transfers, long international conversations on a calling app), rounding error is a small share of the bill. A 60/60 route at a meaningfully lower rate may beat a 1/1 route. Some destinations are only available on coarse increments from any carrier, because that’s how the terminating operator bills upstream. In those cases, you’re comparing providers on rate and quality, not on increments.
The decision rule is simple. Compute the overhead multiplier for your actual traffic to that destination. If the coarse route’s rate, multiplied by that overhead, is still lower than the per-second alternative, take it. If not, the discount was never real.
What to ask a carrier before you commit traffic
Before routing production traffic, get written answers to these:
- What are the increments per destination, and where are they published in the deck?
- Are there connection fees, minimum charges, or surcharges on short calls?
- How is billable duration measured?
- How much notice do you give for rate and increment changes?
- What’s the dispute process, and what CDR format do you accept?
Carriers differ on transparency here as much as on price. When you request a deck from IDT Express or anyone else, ask for increments listed per destination so you can model the effective rate before sending a single call. Whichever carrier you use, make increments a column in your comparison sheet, not a line in the fine print. For a broader comparison of what to look for in a carrier, see our overview of wholesale voice termination providers, and for the fundamentals behind all of this, our guide to how voice termination works.
Frequently asked questions
What does 6/6 billing mean in voice termination?
6/6 billing means each call is charged for a minimum of six seconds and then rounded up in six-second steps. A 7-second call is billed as 12 seconds; a 61-second call is billed as 66 seconds.
Is 60/60 billing always more expensive than per-second billing?
No. It’s more expensive at the same rate, but a 60/60 route can still be cheaper if its rate is low enough to offset the rounding. The break-even point depends on your call-length distribution, and for short-call traffic the 60/60 rate may need to be about half the per-second rate.
What is the difference between 30/6 and 6/6 billing?
30/6 billing charges a 30-second minimum per call and six-second increments after that, while 6/6 charges only a six-second minimum. The difference only affects calls shorter than 30 seconds, but for short-call traffic that can be most of the bill.
How do I calculate my effective rate per minute?
Divide the total amount billed by the total actual minutes your calls lasted. Compare routes on effective rate for your own traffic mix rather than on the quoted per-minute rate.
Why do some destinations only offer 60/60 billing?
Some terminating operators bill their upstream carriers in whole minutes, so carriers pass that structure through. For those destinations, compare providers on rate and route quality, since increments are the same everywhere.
Request a rate deck
Want to model effective rates on your own traffic? Request an IDT Express rate deck with increments listed by destination.
