asterisk users - Jan 2007 - beronet BRI card sometimes not detecting tones

Hi,
I have an asterisk 1.2.9.1 box with a beronet card 
(install-misdn-mqueue). It is working good but some callers can call the 
number to my PBX, hears the intro message but cannot choose which 
extension to call (es: 1,2,...,104, etc) because the card does not 
accept extra digits.
Is there anybody who knows why and how to solve the prob?

TIA

Giorgio