have you configured split-tunneling or tunnel-all for the ip phones?
also you can take captures on the ASA to check if the traffic from your ip phone is even reaching the ASA.
capture cap interface <interface to which the other phone is reachable> match ip host <pool ip address of ip phone> host <ip address of the other end>
"show cap cap" will show you if there is unidirectional communication or bi-directional
