Call flow as below:

Fax Machine -> FXS -> SCCP -> CUCM -> SIP -> IOS -> ISDN

Quick overview of a working Fax/EFTPOS setup on a SCCP Controlled FXS Module.

Note: SCCP supports only NSE switchover negotiation, not Protocol Based. SIP Support both.

Global Fax Configuration

Voice service voip
fax protocol t38 nse force version 0 ls-redundancy 0 hs-redundancy 0 fallback none

SCCP Configuration

sccp local GigabitEthernet0/0
sccp ccm 10.10.10.1 identifier 1 priority 1 version 7.0
sccp ccm 10.10.10.2 identifier 2 priority 2 version 7.0
sccp
sccp ccm group 1
bind interface GigabitEthernet0/0
associate ccm 1 priority 1
associate ccm 2 priority 2

STCAPP Configuration

stcapp
stcapp ccm group 1

Voice Port Configuration

voice-port 0/1/0
compand-type a-law
cptone AU
timeouts ringing infinity

Dial-Peer Configuration

dial-peer voice 7000 pots
service stcapp
port 0/1/0

Regions – Codecs

Regions between the Analogue Devices and the SIP Trunk at the Main site must talk with no codec compression, otherwise faxing handshake is distorted. This extends to the SIP Dial-Peer on the ISDN Router.

Show Commands

Two show commands to use to identify what codec or fax protocol is being negotiated

Show voice call summary
Show voice dsp