VoIP Status dropping registration

[IrvineCAGuy]

I'm trying to diagnose or pinpoint the potential source of this problem:

On a Ver 10 system running on an Intel Atom platform and XP, I get periodically "dropped" from the VoIP registration.
No incoming nor outgoing calls.
This system has been running for a long time with 100% up-time, until recently.

I've installed Ver 12 on an WIndows 7 Pro platform, however here I lose registration even more frequently.
Rebooting either server usually reconnects the registration with my VoIP provider, but not for long.

I am "testing" with using DSL with 60kb up and 4mb down.

So, where can the problem be? My DSL connection? My VoIP provider? Or both computers are not behaving?

 

[DJHiP]

What time do you have set in VoIP Provider -> Advanced for 'Time between registration attempts (in seconds)' ?

And this is on the same Modem/Router, no firmware updates performed which may have affected the NAT timeout?

Lastly what does the Server Activity Log say when the VoIP Provider drops?