Call forwarding to external from internal not reachable

[lan4you]

Hello,
we have a problem with an OXO system on firmware 103/065.001.

At our customer the connection was changed to a S2M IP. Since then, we have the problem that no connection can be established for an internal call on an extension where a forwarding to external is stored. The call number of the call forwarding is displayed briefly and then the message "call ended". The telephone system does not establish a connection.

After a lot of testing, we found out that if the area code of the same location is stored with the call forwarding, then the call forwarding works if we leave it out, then the above message appears. Redirections to cell phone numbers are also no problem.

According to our provider, this is due to the configuration of the system. However, everything worked before the change.

Maybe someone here has an idea what else we can test.

Thank you very much.