9133 "problem": Line gets cut-off whilst dialing (and another call is coming in)

Hi there.

I have this problem and I don't know how to fix it.
This is what happens:

When someone is making and outgoing call (while pressing the digits) and -at the same time- there is an incoming call, then the dialing procedure somehow "resets" so the user has to dial again.

This is a big problem because I have many incoming calls and my users are rushing to dial their numbers in-between the incoming calls!

Any ideas?

Many thanks,

Netizen