Interestingly in the Toshiba KB they have a KB regarding Call-by-Call to limit calls on a PRI with the subject "Fax busy work around for the CTX with PRI only".

Toshiba has a Digitrace utility to capture ISDN messages, but it works about the same as Hyperterminal (or TeraTerm or Putty). That is probably the best way to find out what messages are being sent back to the carrier. You could ask the Charter techs what they see, but the chance is probably pretty low that they will be able to tell you.

Have you tried reducing the B channels to 4 channels?