TGSIP can be used with a TGS Enterprise Ltd member only.
If you subscribe a VoIP to PSTN (=classic telephony) account to a telecom provider, you can reach everyone that has a "classic" phone line.
List of supported features:
-audio with the following codecs: GSM,G711 (ulaw,alaw)
-Address book integration
-DTMF (telephone tones) support using SIP INFO or RFC2833