I had to do a similar task - rather than fiddle around with serial or IRDA, I purchased a GSM module that was designed to work via RS232 with a fully documented interface (Wavecom).And it had analog input and outputs. One less thing I had to worry about!

It also pays to check your controlling code on the PC using VB or Delphi (or whatever!) just so you get the command sequences, and timing correct before trying to debug code on the 8051.