VoIP peering is assisting wireless carriers as well as those of the wired variety.

Here is a link to a good VoIP peering article.

Here is an excerpt from a related release:

InfiniRoute Networks Launches First VoIP Peering Service for Wireless Carriers

InfiniRoute Networks Inc.
(http://www.infiniroute.com) has announced the availability of its Voice over IP
(VoIP) peering service for Wireless Carriers. InfiniRoute's Managed VoIP
Peering (MVP) service, launched in 2004, was the industry's first carrier-
neutral VoIP Peering service to integrate and manage voice and IP routing for
wireline, wireless and emerging carriers.
 

InfiniRoute's MVP service now provides mobile carriers with the ability
to terminate and receive international traffic using cost-effective, high
quality VoIP technology. Carrier benefits include:

      -  Greater access to global markets. Mobile Network Operators without
         extensive TDM networks can now economically reach international
         termination partners without significant capital expenditures.

      -  Lower cost of operations. Mobile networks can leverage VoIP to
         deliver international traffic at lower costs over a scalable and
         inexpensive packet-based infrastructure.

      -  Reduced technology risk. InfiniRoute's proven solution lowers
         capital expenditures associated with emerging technologies.

      -  Faster time to market. The fully managed service, combined with
         InfiniRoute's VoIP expertise, enables mobile network operators to
         enter the global VoIP telecommunications market.