Not much that could be wrong:
1) Check codecs
2) See if the quintum have restrictions on the “allowed” table about what I.P. addressese could send calls to it (very unlike)
3) The call itself. Does the Cisco sends the calls a diferent way (I.E. with a prefix??)
4) Go to the Quintum, enable event log by typing ev +ch and ev +q931
That will put ont he log the attemps. Compare the log of the sucesfull call with the log of the failed calls.