"Your call is being transferred ... please hold"

Hi all,

I've noticed an intermittent problem with my iPhone 3GS when calling people. I ring someone and I hear some ringing (as normal) followed by "Your call is being transferred. Please hold." This message repeats twice and then invaraibly goes to answerphone. Sometimes there will be no ringing beforehand, just silence then the message.

Any time this happens, the person's phone has been on, but they have not got a call or a missed call. Both of us have had signal at the time too. This is not just one person either, it has happened with different people.

Any help?

Thanks

James