Motorola used to have a little box, about the size of a cigarrete box that did that.
The were used for security equipment that needed dial tone for a backup port. The box was installed betwen the cell phone and the security controler.
The little box provided dial tone, you dialed using a single line phone and when the digits got collected, it palced a call using the cell phone.
I am sure that other brands must have the same devices. I sugest checking with people that work on alarms or credit card machines. Those little point of sales machines sitting on the counters in stores.
Good luck!!!!!!