[asterisk-users] MusicOnHold starts magically for no reason

[Markus]

Dear list,

on Asterisk 1.4.21 which is being used in a callthrough scenario - 
callers call via PSTN to a DID coming in via SIP and then dialing 
outbound via DTMF and the outbound calls get routed via some SIP 
termination provider - lately I see that every now and then MusicOnHold 
gets triggered like this on outbound calls:

Started music on hold, class 'default', on 
SIP/outbound-sip-provider-00000002

I do not have any reference to MusicOnHold in my extensions.conf so a 
misconfiguration is unlikely.

Is there some SIP magic that can trigger MusicOnHold on my end?

Thanks!
Markus