Siemens C47H / C475IP setup question

Got this installed with relative ease. Accessing the base unit from the Laptop was wonderful.

Sipgate advised on some port forwarding I needed to do to resolve a one way audio issue. They answered me quickly.

I have put a question now to Siemens, so answer yet. Let me put it here.

I have, following the manual, reassigned the "Display Keys" to "Fixed Line" and "IP". If I press "Fixed Line" I do indeed get an invitation to enter "Phone number:"; but if I press "IP" I get an error message "Not possible".

I have two Sipgate accounts correctly installed, #1 having balance on account to make calls. If I disable account #2 I still get the error, so that is not the problem.

Any ideas?