I was finding the UART to USB a bit of a problem from PIC to Nikia 6301. I have decided to use a Raspberry Pi, this has a USB interface built in and can send AT commands using "cu". So the current plan is to get the pulse dial detected on the GPIO of the Pi and send the AT command from there. This should be a lot simpler than using a PIC.
There will still need to be a bit circuitry to actually ring the phone, I should be able to drive the boost converter etc from the GPIO.
Additional battery power will be needed to drive the Pi, apparently 4 AA batteries is sufficient as the Pi requires about 750ma.
This will all be done with Python scripts.
