answer a call from java application

Hi there,
this question is about the concept with the asterisk,

I am using Asterisk as the PBX for our system, building an application that should support telephony, it should allow a user to initiate a call, hangup, answer an incoming call on-click of some buttons, doing that with known telephony solutions such as cisco it is all provided with the needed APIs that allow such interaction,
but, as the Asterisk is an open source there could be any SIP phone - voip phone - connected to it,

in the situation of the answer action (from the application the application), what actually would the answer do, would it ask the telephone to answer (to open the channel ) via some sort of API for that phone !
Or in the other hand the application answer button sends some thing to the PBX which is then some how handle it and make the phone answers ? (could this be in the asterisk ?)

hope I have made the question clear , plz if this not the right forum tell me
thanks in advance