Thanks for taking an interest.
I've tried on two Galaxy 16+ v3.69SWE on Dualtech TS100 GSM transmitters which imitate a normal PSTN landline.
That didn't work.
I've also tried on a Galaxy Dimension GD48 (unsure of program version but definitely up to date, I got it of eBay UK so it's running British firmare).
Since the good stuff didn't work anyway I thought I'd try something else so I got a crappy generic GSM dialer of Aliexpress.
Both panels/dialers call out to the modem pool just fine but wont negotiate a connection or connect with the modem at the opposite end.
Unfortunately a company called generic mobile have a monopoly on this service in Sweden so I can't just try with another service.