LexicostThe formula
billed minutes = calls × max(⌈seconds ÷ increment⌉ × increment, minimum) ÷ 60- calls: calls in the month; the calculator takes every call to be the average length
- seconds: the length of one call, as the vendor measures it
- increment: the step each call is rounded up to, in seconds: 1 for billing per second (the calculator does not round an average length up to a whole second), 60 for rounding every call up to the minute
- minimum: the shortest length billed for a call, in seconds; 0 when there is none
- ⌈ ⌉: round up to the next whole number
How each vendor turns seconds into a bill
Per second. Retell: Per second; no rounding up per call. Bland: Per second of active call time. Synthflow: Per second, partial seconds rounded up. Seconds are added up across calls; each whole minute is billed.
Steps of a few seconds. Ultravox: All calls are rounded up to the nearest deciminute (6 seconds). SIP minutes: billed per 6 seconds, rounded up.
Whole minutes per call. Telnyx: Engine: rounded up to the minute, per call, with a floor of 60 seconds per call (voice engine). A footnote adds: Cost estimates on the page exclude the 60-second rounding.
A minimum on some calls. Retell, in its rules for AI model minutes: 10 seconds, for calls with a dynamic opening message that end in under 10 seconds. Synthflow: No-answer and hang-up-on-voicemail calls count as 5 seconds; failed and canceled calls are not billed.
Per attempt. Bland: $0.015 minimum per outbound call attempt on Bland's telephony. Retell: $0.005 per dial for batch calls. Telnyx, for its agents: No charge for ringing or failed attempts. Its terms, though, carry a $0.01 surcharge per call of 6 seconds or less (telephony), which its usage-reports documentation applies to accounts where such calls reach a set share of outbound traffic.
Silence. ElevenLabs: Silence longer than 10 seconds is billed at a 95% discount. Retell: Silence and hold time are billed, because its speech-to-text keeps listening.
Voicemail. Bland: Voicemail billed as normal call time. Retell: Failed calls are not billed; voicemail is billed while the agent is active.
Rounding not published. Vapi: Not publicly listed. Its billing docs say: Every minute an assistant runs is billed. ElevenLabs: Not publicly listed. Its help center says how a call is timed: Call length is measured from connection to hang-up or window close.
When a vendor's own pages disagree
Ultravox's FAQ first says calls are rounded to the nearest step, and gives the price: Per 6 seconds: $0.005 each. Later it says: All calls are rounded up to the nearest deciminute (6 seconds). Its own examples round every call up, even calls closer to the step below.
Telnyx, on the phone line under each call: The terms bill calls to the US and Canada in sixty-second increments with a sixty-second minimum, while telnyx.com/llms.txt describes SIP trunking as having per-second billing. On how that line is rounded, the Voice AI pricing page says only that "telephony follows its own billing rules."
Why short calls pay the most
Rounding adds less than one step to any call, so a rounded-up minute is a small share of a ten-minute call and most of the bill for a four-second one. A fee per dial is the same on every call, and a minimum only ever lengthens short calls.
Take an example outbound campaign of 1,000 answered calls: 700 last 4 seconds, because the person hangs up or a voicemail greeting picks up, and the other 300 last 61 seconds. That is 21,100 seconds of talk, about 352 minutes.
Billed per second, that stays about 352 minutes. Rounded up in 6-second steps it comes to 400, since each 4-second call bills 6 seconds and each 61-second call 66. Per second with a 10-second minimum, about 422. Rounded up to a whole minute per call, 1,300 minutes, 3.7 times the talk. Under that last rule the 700 short calls are 13% of the talk and 54% of the billed minutes.
What the calculator does and leaves out
The calculator applies each vendor's own rule to the parts that vendor prices per minute, at its default rates from the cost per minute calculator. For Telnyx that rounds the phone line to the minute along with the voice engine, which follows the terms in the disagreement above. Parts a vendor does not price per minute, such as an AI model it bills per token or a phone line from your own carrier, are left out of the month's figure, and the table names them; a part a vendor lists no charge for, such as Retell's speech-to-text, counts as zero and is not named. Vendors with no published rounding rule, such as Vapi and ElevenLabs, are billed per second; if they round up, the real bill is higher.
An example at $0.10 a minute, all of it billed by the vendor: 2,000 calls of 90 seconds are 3,000 minutes of talk. Billed per second, that is 3,000 minutes and $300. In 6-second steps it is the same, since 90 seconds is exactly 15 steps. Rounded up to the minute, each call bills 2 minutes: 4,000 minutes and $400, a third more, with 1,000 minutes nobody spoke and about $0.133 per minute of talk.
It treats every call as the average length, so the campaign above, entered as an average of 21.1 seconds, bills 1,000 minutes under a whole-minute rule where the real calls bill 1,300. Run short and long calls as two sets and add them. It also leaves out rules that touch only some calls, such as Retell's greeting minimum, Bland's attempt minimum, Telnyx's short-call surcharge and ElevenLabs' silence discount. The cost per minute calculator lists them under "Not in this figure".
Often confused with
- Cost per minute. The price of one billed minute. A lower rate rounded up per call can cost more than a higher rate billed per second.
- Talk time or call duration. The length of the call itself, which billed time starts from and can run past. Ultravox: Billed time runs on the call clock, so it can exceed the recording length.
- Included minutes. Minutes a plan's monthly fee already covers. The calculator subtracts them after rounding, and only from the platform part of the rate.
Vendors cited on this page
Bland AI, ElevenLabs Agents (ElevenAgents), Retell AI, Synthflow AI, Telnyx Voice AI Agents, Ultravox, Vapi. Each profile lists everything that vendor publishes, with its sources. All of them are side by side in the comparison.
Sources
- Ultravox FAQ: how Ultravox bills calls shorter than a full minute, with its table of examples read 14 Nov 2026
- Telnyx Voice AI Agents pricing: the FAQ on per-call rounding and the footnote on estimates and telephony read 14 Nov 2026
Checked against its sources on 14 Nov 2026. Vendor figures on this page open their source when clicked. Lexicost has no commercial relationship with any vendor named on this site.