ITSP Application With the Vigor2600V series router, people not only can make calls to/from other VoIP users across the Internet free of charge, but also can enjoy unlimited worldwide voice calls over Internet via ITSP (Internet Telephony Service Provider) gateway ( e.g. DrayTEL ). ITSP can transport telephone calls over the Internet, no matter whether traditional telephony devices, multimedia PCs or dedicated terminals take part in the calls, and no matter whether the calls are entirely or partially transmitted over the Internet. The useful application implements intercommunication among most popular ways of voice calls like VoIP, PSTN, GSM, etc. Connect FXS to PSTN-PBX As everybody knows, FXO interface of VoIP devices is suitable for connecting to the PSTN-PBX in order to expand the amounts of extensions. The Vigor VoIP routers with FXS interface also provide useful ways to implement the similar utilization. The following are two typical application scenarios. Case1. Calls From VoIP to Extension 1) David dials the VoIP number of Vigor2600V (A). 2) After connection success, presses Linda's extension 610. Case2. Calls From Extension to VoIP 1) Linda presses prefix number (e.g."7") firstly to choose exterior line - FXS of Vigor2600V (A). 2) Then dials David's VoIP number of Vigor2600V (B). Note: PSTN-PBX is widely adopted by most Annex A countries. Because most Annex B countries such as Germany, Norway, the Netherlands have encouraged small offices to deploy ISDN PRI into PBX, PRI PBX are quite popular at offices located in Germany, the Netherlands, Norway.