Making calls is unstable on ZoIPer

Hello everyone. I have a problem is when I want to call on the phone from sofphone sofphone PC is not wanted PC PC performance, received a call to redial. like to call this bait. This error occurs when I use both intranet and internet call out. good network connection speed.
I use Asterisk and FreePBX 12. 13 ZoIPer softphone.
X-lite exam ZoIPer not hear the call.
Thanks everyone.