the chip MH88422 from zarlink is certified to connect to the telephone network over here (that's why they are selling it here).

That does not mean that you can bypass the type approval. All the chip manufacturers around the world make their chips according to certain specifications which does not ,in any way, give you this right.

BTW: Did you bought it from a Zarlink distributor with any certification?

Now coming to your problem, you should check the level and duration of your DTMF signal. Most internal PABXs does not comply to level/duration of DTMF (for detection) as the main centeral office switches. Minimum required duration is 40 msec. I can't remember the level right now.

Regards

IA