Hello Martyn,
I am not sure but I think I know what you mean. snom ONE plays some conform noise on external outgoing calls using the file noise.wav, you can see it in the snom ONE log:
[8] 2012/09/21 18:16:00:Play audio_moh/noise.wav, caching true
I could remove the noise by deleting - or renaming - the file noise.wav from the audio_moh directory (which you can find in the snom ONE working directory (http://wiki.snomone.com/index.php?title=Where_is_the_snomONE_working_directory%3F ) . Make sure you restart snom ONE after you do this, and after this the noise should be gone.