Since we passed to dedicated IP, we have a problem with group call number 9.
Calling 9 from any phone, (phones not included in group, also) is impossible answer the call.
The other groups seems don't have this issue.
Did you find what happens instead of the expected? Did you already check Call Monitor and System Log?
Simply by lifting the handset the phones continue to ring. this only happens by calling group 9
Do you have the same extensions in the working groups? It might be an extension issue which you can check by making an internal call to that extension.