The 3G mobile broadband connection provides the mobile VOIP connection using a portable 3G WiFi router + inclusive 3G USB dongle and a WiFi capable handset with a VOIP client (which switches automatically from the home WiFi network to the mobile WiFi and back again when the home WiFi network is detected).
I'd suppose that your multi-part kit costs more.
I've thought about the 3g->wifi router thing with VoIP, except that verizon's 3G service on my USB dongle is spotty on my commute. Not to mention the fact that I'd need to basically haul a pack with battery to keep the WiFi cloud operational for any device I carry.
But...but but...the 3G dongle is paid for by work...and I could rig up a lighter adapter to power the 3G -> wifi gizmo....
hmm