In order to preserve server resources it is possible to set some upper limits in server configuration.
Realtime processing and timeouts in VoIP protocols leads a sudden drop of performances when bottlenecks are reached.
This setting limits the maximum number of concurrent calls, in order to preserver enough resources to grant adequate quality of service. Proper tuning of this parameter requires proper evaluation of the typical workload. Transcoded calls (one leg of call using AMR codec and the other using GSM or PCM) requires computational power and CPU can become the relevant bottleneck.
As a rule of thumb, keep in mind that one CPU core @ 3GHz can manage about 25 transcoded concurrent calls. For more details refers to section PSAM Appendix C - Server Sizing
Max VoIP accounts
It is also possible to limits the maximum number of configurable VoIP accounts, setting a limit on setting "Max Users".
In our experience, the maximum number of concurrent calls is under then 5% of configured VoIP accounts.
A note remembers you how much users you created and the maximum numbers available, both in groups list and account list as shown in the following image: