I recommend you to take a CCM detailed trace on the CCM where the phone is registered, then make a call and gather the trace. Most likely you will see the Create Connection message to one IP that you can reach but then a Modify Connection message to an IP that you can not ping, probably a bind on the GW will help.
I also recommend you to take a 'show voip rtp connections' while making the call. You should see a local side and a remote side. Then from the GW where you are taking that command perform a 'ping remote_IPAddress source local_IPAddress' with the IP addresses that you saw on the show voip rtp connections and see if you can reach them.
Most likely when you press the hold and then resume to modify the connection to an IP Address that you can ping, that is why you fix it by doing that.
