Each call is cut off after 3 hours which is set for security reasons and that is normal for low cost voip networks. It is by design to make sure calls don't hang on the system.
As a residential service, it does not affect normal usage patterns. You can make the call again, with no issues.
Our low cost residential plan has this system wide limitation to prevent calls from not being hung up properly. We do have a small business wireless plan that may suit you better.