Liquid error: undefined method `name' for nil:NilClass
I believe the issue could be with the outbound assignment, on the gigaset if you go to lines I think it is, you will see "incoming", "outgoing" and "PSTN" can you please make sure the outgoing is using the sip line?
I do not have a gigaset on me at the moment but I can have a look at it in the morning. Please have a look and see if you can find that setting.
I am glad you got sorted!