Check your computer hasn't got any dialling patterns set up, like dialling 9 for an outside line. I would give you directions to the settings to chance this, but I can't access Control Panel here. It might be under Telephony or Regional Settings.
Also if your phone line is ex directory, your line may be withholding your number automatically, most sevice providers will not accept an annonymous access call, to over ride this prefix the dilers number with 141