Well it seems that the PiPhone premade project was all good to go on a Pi2 with an older version of Raspbian , but it has issues with:
a) Raspbian Stretch and
b) the Pi3b.
The touch screen doesn’t want to work on a Pi 1 A or the Pi3b. Adafruit and another friends in the know have said it’s a bit of a challenging one to get going quickly.
Just for the moment I thought I’d just get 3G going using a Pi3b as a router using NAT. That works fine. Whilst it’s doing that phone calls come in, but I can’t pick the calls up as the 3G data connection blocks the serial.
At this point it looks like I could play with hardware flow control on RTSin pin. There is also an other option for software flow control somewhere in the manual.