Hi Abbas,

PSTN connectivity is generically referred to as ** VG - voice gateway functionality, offering a gateway for voice over IP (VoIP) calls to, and from, traditional analog or digital PSTN or private branch exchange (PBX) calls. You can use a router voice gateway to connect to PSTN central office (CO) switches, private branch exchanges (PBXs), Key Systems, time-division multiplexing (TDM)-based interactive voice response (IVR) systems, traditional TDM-based voice mail systems, and any other legacy (non-IP) voice processing or telephone equipment.There are also Station Voice Gateways like the VG200 Series that provide Analog station (Phone,Fax etc) connections in CCM and CME systems.

Cisco Unified CallManager Express Solution Reference Network Design Guide

http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_implementation_design_guide_chapter09186a00805f0758.html

Cisco Access Analog Gateways

There are two categories of Cisco access analog gateways, trunk gateways and station gateways.

Access analog station gateways

Analog station gateways connect Cisco Unified CallManager to Plain Old Telephone Service (POTS) analog telephones, interactive voice response (IVR) systems, fax machines, and voice mail systems. Station gateways provide Foreign Exchange Station (FXS) ports.

Access analog trunk gateways

Analog trunk gateways connect Cisco Unified CallManager to PSTN central office (CO) or PBX trunks. Trunk gateways provide Foreign Exchange Office (FXO) ports for access to the PSTN, PBXs, or key systems, and E&M (recEive and transMit, or ear and mouth) ports for analog trunk connection to a legacy PBX. Whenever possible, use digital gateways to minimize any answer and disconnect supervision issues. Analog Direct Inward Dialing (DID) and Centralized Automatic Message Accounting (CAMA) are also available for PSTN connectivity.

Cisco Access Digital Trunk Gateways

A Cisco access digital trunk gateway connects Cisco Unified CallManager to the PSTN or to a PBX via digital trunks such as Primary Rate Interface (PRI), Basic Rate Interface (BRI), or T1 Channel Associated Signaling (CAS). Digital T1 PRI trunks may also be use to connect to certain legacy voice mail systems.

Cisco Unified Communications SRND Based on Cisco Unified CallManager 4.x

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a00806e8c11.html

Understanding Voice Gateways Cisco Callmanager

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801eff84.html

Hope this helps!

Rob