Definition
Concurrent calls are the calls that are connected at the same moment. The count is a peak at one instant, not a daily total, and it sets how many callers a trunk, platform or agent can serve at once.
How concurrent calls work
A call counts from the moment it connects until it ends. Ten calls of ten minutes each, started one after another, are one concurrent call. The same ten calls started together are ten.
To estimate the average, multiply the rate of new calls by how long each lasts. If 120 calls arrive per hour and each runs 4 minutes, the average load is 120 times 4 divided by 60, which is 8 calls at once. Real traffic bunches up, so the peak is higher than the average. The classic way to size lines for random arrivals is the Erlang B formula, which gives the share of calls that would be blocked for a given number of channels. A simple rule is to take the average, look at your busiest hour and add headroom.
The limit can sit in several places at once: the carrier’s channel count on the SIP trunk, the voice agent platform’s plan, and any capacity of the speech models and LLMs behind the agent. The lowest of them is the real ceiling.
Why it matters for voice agents
A person handles one call at a time. A voice agent can run many calls in parallel, so the ceiling becomes a number you choose and pay for, not the headcount in a room. That changes how you plan an inbound line: a morning rush or a campaign mention that sends fifty callers at once is fine at a limit of sixty and a failure at a limit of twelve.
It also sets the pace for batch calling, where a campaign can dial only as many numbers as there are free slots.
On Cartesia, the pricing page lists concurrent calls per plan: 8 on Free, 12 on Pro, 20 on Startup and 60 on Scale, with a custom limit on Enterprise.
Example
An illustrative sizing: a clinic expects 90 inbound calls an hour on Monday mornings, averaging 3 minutes. The average load is 90 times 3 divided by 60, which is 4.5. Peaks several times the average are common, so the clinic plans for 12 and checks the trunk allows as many.