Dont play moh on external calls


when we receive external calls through DAHDI/SIP/IAX, the caller doesnt hear the music on hold from the Inboundroute or the ringgroup. If i make a internal call, it works fine.
after i put a Answer in extensions_custom.conf, it works too.

[from-pstn-custom] exten => _X.,1,Answer

i dont want to answer each external call. is there any other better solution?