Voice Gateway · Analog
Generate clean IOS-XE voice-port and dial-peer configuration for FXS analog stations (fax, paging, door phones, modems) and FXO analog trunks to the PSTN. Includes signaling, disconnect supervision, T.38 fax relay, and CUCM wiring.
Analog never died. Customers still want a fax line on a copier, a paging amp on the warehouse wall, a 911 elevator phone, or an ATA in a guest room. This tool builds the FXS or FXO config blocks that 90% of small Cisco analog deployments need, plus the dial peers to wire them to CUCM or to the PSTN.
FXS ports run signaling out to the analog device — they provide dial tone, ring voltage, and battery. FXO ports do the opposite — they look like a phone to the carrier and seize the line by going off-hook. Get the direction wrong and nothing rings.
For paging and door phones, PLAR (Private Line Automatic Ringdown) is what you want — go off-hook, the gateway dials a fixed destination. For fax, T.38 relay is the modern answer; passthrough only when both ends are local and the path has no transcoding. Disconnect supervision matters on FXO — without it, calls hang the line and your PSTN trunk goes dead until manual reset.
The generator produces a voice-port stanza, a dial peer (POTS for the analog side, VoIP for the CUCM side), and any required helpers like translation rules and fax-relay settings. CUCM-side configuration is provided as a checklist since CUCM is configured via web GUI, not CLI.
FXS provides battery, ring voltage, and dial tone to the connected device. Typical settings include impedance (set per country), cptone (call progress tones — busy, ring back, dial tone), and a station-id name/number. For paging or door phones, add connection plar opx (or plain connection plar) so the device dials a fixed extension when it goes off-hook.
FXO loops the carrier line. The biggest sin in FXO design is forgetting disconnect supervision. Without it, the gateway never hangs up after the far end disconnects, the trunk goes into permanent off-hook, and the next inbound or outbound call fails. Always order battery-reversal from your carrier when possible; fall back to disconnect-tone or power-denial as last resort.
T.38 is the standard. The gateway negotiates V.21 preamble, switches the call from G.711 to T.38 fax relay, and the fax pages travel as IP packets. Keep CCS low through the fax path and avoid transcoding mid-call. Some MFP devices (HP, Ricoh, Konica) need T.38 version 0 forced; default is fine for most.
FXS connects to a station (phone, fax, ATA). FXO connects to a trunk (PSTN, Centrex, PBX). Mnemonic: the port is named for what's on the other end of it. FXS-to-station, FXO-to-office.
Disconnect supervision is misconfigured or your carrier doesn't send it. Order battery-reversal from the carrier, then add battery-reversal answer on the voice-port. If the carrier won't supply it, configure disconnect-tone with a sample of the local cpt-tone and add aggressive call-progress-tone detection.
Sometimes, mostly no. T.38 needs jitter under 10ms and zero packet loss across the fax window. Internet QoS is best-effort. For reliable fax, terminate analog locally and send T.38 only across an MPLS or SD-WAN with voice priority queues.
NIM-2FXS (2 ports), NIM-4FXS (4 ports), NIM-8FXS (8 ports). All are channelized — every port is independent. PVDM4 DSPs are still required if you transcode the analog leg to G.722 or Opus on the IP side.
Only if you have a voicemail system that supports analog MWI (FSK or stutter dial tone). CUCM + Cisco Unity Connection supports both — set the MWI mode in the gateway voice-port stanza and configure the MWI device pool on Unity.
Bogen 5W amp on port 0/1/0. PLAR to extension 7000 (paging route point on CUCM). When the receptionist dials 7000 from any phone, CUCM routes to the gateway, the FXS port goes ring → off-hook simulated, and the amp keys the paging zone.
NIM-4FXO at slot 0/2/0–0/2/3. Ground-start signaling (the carrier is a class-5 switch). Battery-reversal disconnect. Trunk group with circular hunt. Outbound dial peer matches 9.T (9 + any digits) and strips the 9 before sending to the carrier.
Copier MFP with built-in fax on port 0/1/2. Extension 5500. T.38 fax relay version 0. ECM enabled. Dial peer to CUCM (which has a fax-pattern translation to dial 9 + outbound DID). Tested with a 4-page test sheet at 33.6 kbps — clean send.
Drop your email and we'll send your generated FXS/FXO config plus a CUCM analog gateway checklist as PDF.