Is there a softphone running in the iOS background?

Question

SIP

Good day!
A few years ago I purchased an ip intercom with SIP support. And I ran into a problem because of which, in principle, the use of vulture does not make sense. On an iOS phone, I can’t find a really working softphone in standby mode. Although one solution works flawlessly is 3cx phone but it does not support video calling. The fact is that other solutions stop receiving calls from the sip server when the application is closed, and even more so after the iPhone is restarted, that is, you constantly have to monitor whether the application is enabled or not. Tried different options both free and paid. Tell me if there is such a solution running in the background in iOS?