Hi,
Sometimes pinging isn't an accurate test for the VPN. The ACLs that allow traffic may not be purely IP based, but also on protocol. As well, your ACLs may be from the phone to the CME IP, but not from phone to phone.
A good way to test if you're getting packets or not is to either go to the phone webpage or press the green '?' mark twice quickly, and it will bring up a screen.
If this is CME, you may find that adding 'mtp' under the ephone configuration helps as well for a workaround.
hth,
nick