The only this to do if selling anything online or anywhere that you provide a number is to buy a PAYG SIM & put it into an old handset. Not of much use to the OP now, I know. I never answer private numbers. I recently discovered UPC Anonymous Call Rejection, a handy facility which works in such a way that the phone will not even ring if the caller has a witheld number & instead gets an automated message saying 'We're sorry, the party you are calling does not wish to accept calls from witheld numbers - if you wish to reach this party please unblock your number & try again.' Just dial *90 & call to activate & #90 & call to deactivate. The only downside is that when you activate it, call answering & call waiting are also automatically activated in the process. It would be great if mobile operators would provide this facility.