Using an ACL to block RTP will also prevent fax calls as well.  The DSPs on the ATA use RTP to detect fax tone, which is what initiates the switchover.  If you block RTP, then fax calls will also fail.

I'm not sure you are going to find a way to do this.  In order for a fax call to be negotiated (passthrough, t38, etc), a voice call must first be established. You won't be able to differentiate the two in order to block voice calls.

It's not enough to just change the dtmf method under the voip dial peer to AA.  You need to make sure that whichever dtmf method you are using on CME also matches CUE.  What dtmf method do you have configured under ccn subsystem sip on cue?  For simp...