SIP is IP trunking.
There are currently two "flavors" available on BCM.
SIP (session initiation protocol) or H323.
I haven't worked with SIP too much yet, but one limitation I recently ran into with SIP is an inability to pass DTMF tones.
So, you do need to use VoIP trunks.
The type is up to you, but personally, I'm still partial to H323.
Good Luck!
-SD-