Unable to connect the call in background or terminated state from second call

[wirelessmundi]

How to achieve this to make a call while the receiver app is in the background? The first call okay. But it hangs and if I return to the foreground the rest of the calls fail.

18:29:14.281522+0200 runningboardd Assertion 32-143-27198 (target:[application:1958]) will be created as active 18:29:14.283117+0200 runningboardd Attempting to rename power assertion 42380 for target application to application;32-143-27197:CSDVoIPProcessAssertion-xxxx(VoIP);32-143-27198:CSDVoIPProcessAssertion-xxxx(VoIP) 18:29:14.283963+0200 runningboardd Finished acquiring assertion 32-143-27198 (target:[application:1958])

18:29:27.247808+0200 runningboardd Invalidating assertion 32-143-27197 (target:[application:1958]) from originator [daemon<com.apple.telephonyutilities.callservicesd>:143] 18:29:27.362807+0200 runningboardd Attempting to rename power assertion 42380 for target application to application32-143-27198:CSDVoIPProcessAssertion-xxxx(VoIP)

please let me know