btyers is corrrect on both counts. As far as I can see there are two problems.

1. The phone connecting to the other network when the home network should still have coverage. This may be because the network is setup wrong, the phone is too "agressive" in switching to the other network, or both.

2. Calling working in 2G but not 2.5G
That just doesn't make sense. I can't see any reason why having 2.5G coverage should make any difference unless something is severely misconfigured somewhere. And even then I can't imagine how they've manage to mess it up but leave calling working for 2.0G

I'd suggest you set network selection to manual, (people might want to do this even if there is no problem depending on the cost of roaming) that way the phone shouldn't change network without your "permission". And then contact BOTH networks and explain the problem. Since you don't know where the problem is you want them both to investigate.