I tryed to configure rtp header compression in the policy attached to FastEthernet and failed-
Col(config-pmap-c)#compression header ip rtp
%FastEthernet0/0 does not support IP header-compression
Why it is so?
With native encapsulation, nothing.
In theory you could run a VPN between routers, and run compressed IPsec on it to gain LZH compression (cRTP doesn't work with ipsec), but it seems an overkill.
Hope this helps, please rate post if it does!