I have clients using the gamut of equipment from old POTS line transformer couplers up to the latest VOIP wireless boxes. The cheapest and very decent sounding is a laptop running Skype.
The most fool proof (I got in trouble for saying jock proof) is something like the Comrex Access w/ the add on mixer. Plug in a mic, headphones, hit dial and you are on the air. The lesser amount of cords, cables and knobs the better.
I have one client that is looking at a Comrex OPAL for interviews and low tech remotes. It is basically clicking a link sent in an email and you are on thru a phone or laptop browser. I am very interested to see how that works out for long remotes andevents.
Your choice needs to strongly consider the ability of the talent and availability of last mile service (cell, ISDN, WIFI, POTS, etc).