Hi, Can anyone help me please?

I have a Onetel OT2500 phone and I would like to turn the external volume down on the answerphone so that when someone calls and the phone diverts to answerphone I don't have to hear my outgoing message blaring out at full volume or the message that is left. It says in the user manual that the default volume is 3, but there is no mention of how to change this. Thanks.

Kate, January 2010