I am experiencing exactly the very same problem right now.
The only difference is that the VOIP provider gets the Agent EGW sv.1.88.
This has caused a mess with the VOIP provider's Answering System: it does not work anymore.
Furthermore the phone number results as "non existent, unreachable" causing a failed call.
The setting remains on the VOIP provider for longer than the default 180 seconds:
it appears that the program causes a registration of the device for longer than 3 days!!!!
So my question would be: it is a Zoiper's bug or the VOIP provider?
R