NFAS only support T1 PRI

Restrictions for Configuring NFAS with D-Channel Backup

Restrictions are described in the Restrictions for Configuring ISDN Voice Interfaces, page 4. In addition, the following apply:

•NFAS is supported with only a channelized T1 controller and, as a result, is ISDN PRI capable.

•The router must connect to either a 4ess, dms250, dms100, or National ISDN switch type. Table 47 shows applicable ISDN switch types and supported NFAS types.

Reference:

http://www.cisco.com/en/US/docs/ios/12_3/vvf_c/cisco_ios_isdn_voice_configuration_guide/isdn06.html