[reSIProcate] Doubt regarding how the Presence information of clients is achieved in SIP

[Rajkumar D <rajkumar.d@xxxxxxxxx>]

I want to know how the presence information of all the contacts is got 
in an application like Push to talk over Cellular.

I know it uses SIP for this, but in what format it is got and from where 
the client gets its contacts is not known.

Bye !