> I'm using the CML FX633 Call Progress Tone Detector chip at the
> moment to do this - it indicates if there's a valid tone (ring,
> engaged, etc) on the line - (you have to check the cadence in the uP
> software). But it's too expensive here to make the project feasible.
> I've tried to build my own filter (400Hz), but without much joy. It
> definitely lacked the noise immunity & gain control of the i.c.

If you are trying to detect a single tone, an edge detector with a PIC can
do so fairly easily.  If you're looking for two tones which are in a close
harmonic relationship (e.g. 400:450 = 8:9) you may be able to use pattern
matching on the output of an edge detector.

If you're trying for the general case, a PIC is powerful enough to handle
it with an edge detector for a reasonable number of tones (e.g. PIC16C620
picking up 40ms DTMF tones) but I don't know if my employer would be too
happy if I gave away all my secrets.  Sorry.