I don't have an iPhone, so I did Zoiper testing on an Android. My initial test totally failed, it didn't even ring the Android and eventually timed out. I found Zoiper requires you to send the call to the UserID. Every other softphone I used ignored this field. Anyway, send the call to the AuthUserName instead of zero. In my example I used OBi200, so the fork should look like this:
ph,sp2(OBi200@xx.xx.xx.xx:5060)