Paul,
Simply put, he's using Gamespy in FSX. It allows voice communication on a host specified frequency in game. Again, I recommend using FSHost (in concert with Teamspeak if you want voice) so both sims can participate.
The very first thing I noticed, as soon as I purchased FSX when it was first released, was that I could no longer connect to Multiplayer within the Sim, with the same ease that FS 2004 allowed me to do with FS Host....

...!
From that day on, I have always used my FS 2004 for all my Solo and Multiplayer flights, which is a shame, as FSX was/is probably very good for Multiplayer Flights?...
..its just the Multiplayer interface, or lack of it, in FSX, which creates the difficulty...(ie: Gamespy?).
Paul...G-BPLF..FS 2004+FSX...FS Nav... 8-)...1
In the olden (Dial-up) days...

..., using FS 2002, we could host/connect with each others Flights using our individual IP Addresses..it worked reasonably well (Providing the Address didn't change during the Session!)...

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.