Zoiper for iOS uses the default expiration times for the respective transport types, which are:
- UDP: 60s;
- TCP/TLS: 600s.
The reason for that is the nature of the iOS system, where if you do too many request for certain amount of time the operating system will kill the application. On the other hand, if the requests are too rare, the opearting system will suspend the application in background causing it to not receive any calls.