This is not really an android question, or even specifically a ZoiPer question, but I am hoping some smart people here can help, as this behavior has already defied many attempts at explanation.
Basically, I have a couple of handfuls of VoIP accounts. About half of them "work" (meaning I can use the account information to make calls), and half of them do not (meaning I am completely unable to even register with a sip server). I know the credentials are valid - in many cases, I can even log into the associated web sites with them (assuming they don't have separate sip and account logins). When I say I can't register, that means I can't register: ZoiPer, a number of other softphones, and even real hardware VoIP phones.
To give real examples, I have a Callcentric account that works perfectly form anywhere, and a Localphone account which I literally have not been able to register from anywhere for several months. I have spent hours trying to track this down (as have their support people), and their last offered answer is to blame my network (same problem if I am using a VPN, so that network routers and firewalls can not rewrite the packets before they exit the local network). They also indicated the fact that they don't proxy media might be a source of the issue.
In ZoiPer, attempts at registration eventually die with a 408 (timed out) message. I am wondering if anyone here might have an idea of what is causing this, or offer some advice about troubleshooting.
Thank you in advance.