Unfortunately, the receiver is not a PIC, and the transmission standard calls for 1200Bd. I did some more research and your right.. it is the V.23 standard modem.

I'm not keen on a square to sine wave converter.

There is one other path I could take... and that is to have two free running oscillators, one at 1300Hz and the other at 2100Hz and then use the PWM or other output to switch between to the two....

I could use three pics in total... one each for the mark and space.. and the other to switch between the two.. although this seems a bit extravagant.