My USR Courier V.Everything with X2 upgrade now connects just fine
to the Lucent modems on PM3's. I did find that ATX6 would often
result in an almost immediate hangup when the USR heard the v.8bis
tone. It would report the VOICE answer status and hangup. Curiously
enough, the problem occurred only while making long distance calls but
not for local calls. The "VOICE" detect and hangup problem applied to
a non-X2 Courier as well.
So, I would suggest that the init string for the Courier modem avoid
X5 and X6.
FWIW, my init string for v.34 or X2 is AT&F1t&m5&k3X7.