It depends. The IP phones have to register with something to make
calls.
Which typically is either your VOIP provider or something like asterisk.
The benefits of Asterisk are that it is a full featured PBX system (
multi line, voicemail, conference calls, etc) and is completely open
and configurable.
Much like Linux is used to replace commercial Unix, Asterisk is used to
replace a high cost PBX system.
Asterisk allows you to have multiple VOIP providers at the same time to
allow least cost routing of calls. Additionally it can be used to link
two remote locations across the internet, as a platform for any sort of
phone based project, or to bridge unrelated telephony technologies.