Failed to forward incoming call to external number in p-series pbx

Dear support, lately I have had a regular occurrence that customers who call me when forwarding to mobile do not forward. The call comes in but I can't hear the other end even after 1 minute so I put on and on a new call it happens again and say I'll call them back when I'm in the office. This is useful because a call forwarding is intended to be able to take calls. I've already called my provider for an investigation but nothing comes out so I have to look for it in my S20. Hopefully you can help me with a solution. Greetings Michel Kocken Meps.