Asterisk "MOH" Debugging

Hello, I installed Asterisk 1.6.1.6 for a client, but I have a problem with Music On hold. If I force it through the dialplan (like, for example "exten => 999,n,Dial(SIP/21&SIP/22,,m(default))"), the caller hears the music, but when an active call is being placed on hold or transferred, nothing happens in asterisk CLI and no music is played and the caller hears a blank until it's picked up again, either by the party that put it on hold or by the person to which the call was transferred. I need help in solving this problem. Please note that my budget for this bug fix is small. Thanks, David