In an interview with Japanese website 4Gamer, Kojima mentions that while in Ground Zeroes you could only communicate with Kaz, in The Phantom Pain you’ll also be able to contact Ocelot. This marks the first time the famous character is available as a radio contact.
Also, Kojima mentioned you will be able to do more with the radio that in Ground Zeroes, but what he exactly means by that isn’t clear yet.
Kojima did not mention any other contacts beside Kaz and Ocelot, but he could still be keeping them a secret. Still, pretty exciting news. Now we will be able to hear him go on about sliding long bullets into greasy chambers any time we want to.
Source: 4Gamer, Junker HQ