Do you mean that you cannot hangup a call, regardless if it was an incoming call or an outgoing call ?
What phone model is this ?
What version of android is this ?
Are you able to switch to a different program at all ?
If you cannot switch to a different program and can only reboot the phone, the issue is not Zoiper, but a bug in your phones firmware. (We have seen this on HTC one m8 before).
You can somewhat work around it by switching to the "external Audio driver (java)" in Zoiper, but it will still not be 100% reliable