SS7 stands for Signaling System number 7. It is a messaging protocol used in telephony networks worldwide which handles call control and provides many other services. SS7 typically does not operate at the network edge, rather, it is used for communicating between switches and other network elements inside the telephony network. To learn more about SS7, there is a good tutorial a href=http://www.iec.org/online/tutorials/ss7/> here.
Basically a VoIP gateway only needs to support SS7 for applications where it is desired to connect from the IP network directly into the SS7 network. Service providers that are partnered with other phone companies and Internet Telephony Service Providers ITSPs would likely use such a gateway.