Asterisk - detect and count DTMF

We need a coder who is familiar with programming for the Asterisk platform to create a module for the 1.2 series and the 1.4 series which will detect and count DTMF key-presses made by callers, and store the count in a channel variable accessible by our existing AGI scripts. This must not affect any other running modules or interfere with any other channel variables being used.