Here's my stab at it.
The phones are talking to the 3300 ok.
When a call is made, the phone is actually setting the call up through the 3300. The 3300 signals the other phone to ring and then drops out after the call is established. The phones then talk to each other - not through the 3300. ( I'm talking strictly IP phones here.)
I suspect the problem is that the phone are blocked from talking to each other for some reason. Most likely an IP routing issue.
So here is what I would need to know:
(1) the IP, subnet mask and gateway of the 3300
(2) the IP, subnet mask and gateway of thephones for both sites
So, check these things and I would suspect you'll find the problem there.
Ralph