Difference between gateway and router

A gateway acts as a conversion from one protocol to another or in the case of Voice over IP (VoIP) from the VoIP network to the POTS network. 

A router works by looking at the IP address in the data packet and decides if it is for internal use or if the packet should move outside the network (to the WAN). In a VoIP conversation, you would have a gateway that works for any calls that can not be carried via IP and it moves them to a POTS connection. You generally need both.

For more information:

• Learn more about  IP telephony gateways and their important role with unified communications in our IP telephony gateway primer.
• What is a VoIP trunk gateway? Find out in this WhatIs.com definition. 
• Learn how VoIP works in this tutorial.