I had the same problem with Axon. When I was beta testing 1.20... I was using "Freedigits" as a provider and it would only answer a few calls within a certain period of time and then it would simply stop answering. After discussing the problem with someone on the NCH development team, the "bug" (which is more of an issue of the service provider not correctly telling Axon/IVM when to re-register) was corrected in Axon to my knowledge.
So I have a feeling if you setup the line in Axon it will work, and that IVM simply hasn't been updated yet to correct this problem. Axon is a free product from NCH and it won't interfere or cause your voip line to be answered with IVM any differently, but hopefully it will solve the problem of registration time outs.
In case you're wondering how to set it up..
1) Install Axon
2) Create an "External Line" and use the details from your service provider
3) Create an "Extension" (or use a pre-existing one and obtain its username/password)
4) In IVM remove the SIP account you setup with your provider
5) In IVM add a new SIP account, and fill in the details from the extension you created and use localhost as the server address
6) If you haven't already done so, set the external line you setup to ring on the extension you setup with IVM
The result is, Axon is the program connected and registered to your voip service, and incoming calls will first go to Axon which will direct them to IVM. There are also added benefits of having Axon manage the phone calls instead of just IVM as you will see once you install the software.