" The basic function of a gateway is to translate between different types of networks. In the data environment, a gateway might translate between a Frame Relay network and an Ethernet network, for example. In a VoIP environment, voice gateways are the interface between a VoIP network and the public switched telephone network (PSTN), a private branch exchange (PBX), or analog devices such as fax machines"  source " Cisco Voice Gateways and Gatekeepers"

while the trunk is a logical link used for sending or terminating a voice call

for example you can have a SIP trunk that terminate on a voice gateway

or you may have a voice gateway define as h323 that connect to PSTN using for example ISDN

and you have an other PSTN link through a sip trunk direct from the callmanger to the Teleco using SIP trunk between the CCM and the Telco

these only simple possable scenarios to show the differences between a trunk and a gateway

HTH

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