Digital Voice - dropped calls on mobile phone

I moved to Digital Voice a couple of months ago.
My equipment is :
FritzBox 7530
Panasonic handset base station plugged into FritzBox
3 further Panasonic handsets
iPhone14 with Fritz Fon app.

Generally this all works well, with one small "glitch" :

If I answer an incoming call (to my landline number) on the iPhone, the calll connects , but "drops" after about a minute. Calls which I answer on the Panasonic handsets do not suffer from the "drop" issue.

Any suggestions / fixes ??