Transferring a Call Causes Call to Drop

I'm having a problem where our receptionist using a T28 tries to transfer a call to a user who is also using a T28, the user is currently on a call. Instead of the forwarded call going to call waiting the original call he was on gets cutoff and the new call is sent to his phone.

This also happens from time to time if one receptionist is on a call and a new call comes in it will cutoff her existing call.

All phones are T28 using the Elastix PBX software.