Conference bridge

Hi all,
I have setup a conference bridge with pin on freepbx and asterisk 2.2.0-22,
it works well locally with the pin, but US people calls the conference bridge, the call is patch but when the user enters the same pin, asterisk received nothing and plays invalid pin.
note: I am situated at Mauritius
any helpful suggestion will be great thank you.

What channel technology?

If DAHDI, analogue or ISDN?

If VoIP, how is DTMF being encoded?

If inband, what codecs are being used throughout the complete path?

its a dadhi, ISDN pri line

My guess is that either the voice is being compressed heavily on the trans-oceanic link, or echo-suppressors on the line think that Asterisk is talking and have muted the remote end.

Unfortunately, if it is one of these, the caller would probably have to pay a lot more for the call to get a connection with them disabled.