well, you are juggling two things here.
LCS for PBX to LCS integration and IM. (PC to phone calls or PC to PC calls, etc)
Presense can do the same as LCS, but has more features integrated into the CallManager platform. (is the phone off hook, on hook, etc)
LCS and Presence can federate with each other, Microsoft only allows one way communication. Example: If Bob is Presense, and has Susie in his "buddy list" Bob can only see that she is online, Bob does not know if Susie is on the phone, etc.
If you are to change the roles, Susie would be able to see if Bob was on the phone.
Cisco plays nice and does 2 way federation, but MS does not play nice and only allows one way.
Between the two systems, you could create a SIP trunk between. You would need to create dial plans on each to transfer calls. Think of it as two seperate PBXs with a trunk between.
If an LCS person wants to make call, you could have a rule in LCS to transfer the call via the PBX T1 trunk or via SIP handoff.
If you have a softphone, and it's SIP based, it has to register with a SIP proxy or SIP server. It doesnt just magically work. If have IPC, it registers with CallManager. CallManager manages the call, not LCS.
It's a little confusing what you are trying to do, but hopefully this helps.
cheers
