From the testing of the last few days, the freezing bug seems still there with some new behavior. The bug has a much higher chance to be reproduced if I let it run for at least six (maybe less) hours and then switch the SIP provider. 

After six hours of running in the background, the application may show the following behavior when switching providers:

1. It may revert to the the first entry of the providers. Switch provider again at this time, the application freezes.
2. The application freezes directly. 
3. The application may work fine after switching provider. 
4. If I put a wrong SIP server in the setting for one provider, the application seems to have a higher chance to freeze when switching providers.

If you cannot reproduce the bug, then put it aside at the moment. It is not something urgent and there could be something else going on my PC. Thanks for the great work!

So I found a way that may ease the reproduction the bug. With the latest v2.28 or v2.29 beta, add two SIP providers, Voxalot and Gizmo. (or any other SIP providers, the result is the same.)

Follow the following steps. 
1. Voxalot is registered and active.
2. Call the Voxalot number from another sip phone.
3. After Phoner rings, either hang up the caller, or pick up the call at Phoner then hang up, that doesn't matter. (The caller ID shows as xxxxxxxxxx, a 10 digit US number. I set the caller ID via SIPBroker)
4. Switch the SIP provider to Gizmo. You will see Phoner still reverts to Voxalot. And the debug window can prove that it really re-register to Voxalot but not Gizmo. (Call the Voxalot number again at this time, I see that Phoner now shows the caller ID as (xxxx)xxxxxx and with a German city name.)
5. Switch provider again, or exit Phoner. Phoner will freeze. 

I installed a fresh copy of English Windows XP SP2 in VMWare and did the above test, the result is the same except that in step 5, Phoner will not freeze, but when switching providers in step 5, it always reverts to Voxalot whatever you do. But it will not freeze.

In step 2, if it is Phoner that originates the call but not be called, the bug will not appear in the following steps.

I've been using sipper.dll from v2.23 and phoner.exe from v2.24 that does not appear to have this bug. However, this old version seems to lose registration from time to time. 

Hope the above helps.

Mail sent. Thanks.