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A.Calls to the PSTN can be made via the use of FXO or T1/E1 trunk ports.
B.Calls to the PSTN can be made via FXS or T1/E1 trunk ports.
C.Analog and IP phones can be connected to the VoIP network via FXO or T1/E1 trunk ports.
D.Calls between analog phones that are attached to the FXS ports in a VoIP network can be completely processed by voice-enabled routers.

A.The FXS interface allows an analog connection to be directed at the public switched telephone network （PSTN’s） central office.
B.The FXS interface connects directly to a standard telephone，fax machine，or similar device and supplies ring，voltage，and dial tone.
C.The FXS interface connects directly to an IP phone and supplies ring，voltage，and dial tone.
D.The FXS interface connects directly to ISDN voice channels.