I have installed my own Asterisk server.
I have an IAX2 as well as a SIP account. I can place calls from both accounts, but with Zoiper on WP8.1 I can only receive calls on my IAX2 account, the SIP account won't ring, the call would go straight to voicemail (after the timeout specified in dial).
My extensions are set as follows:
exten => 99,1,Dial(IAX2/iax.lumia,30)
exten => 99,2,Playback(tt-weasels)
exten => 99,n,HangUp()
exten => 44,1,Dial(SIP/sip.lumia,30)
exten => 44,2,Playback(tt-weasels)
exten => 44,n,HangUp()
I can't see a difference.
Best regards, Roland