Damit könnte es funktionieren, den Fax-pass-through Modus über SCCP zu aktivieren:
(auszug aus der offiziellen Doku)
For a Cisco ATA that is registered to a Cisco Unified CME system to participate in fax calls, it must have its ConnectMode parameter set to use the same RTP payload type as the Cisco voice gateway that is performing the fax pass-through. Cisco voice gateways use standard payload type 0/8, which is selected on Cisco ATAs by setting bit 2 of the ConnectMode parameter to 1. For more information, see the " Parameters and Defaults" chapter in the "Cisco ATA 186 and Cisco ATA 188 Analog Telephone Adaptor Administrator's Guide for SCCP" (version 3.0).