No, that is not necessary. The SIM card connects to the telephone exchange. So you can be reached or called on several numbers, fixed and/or mobile, and even time-based. An example: your business mobile number is on your business card. You don't want customers to disturb you at the weekend or on holiday. So you forward your business number to the fault-clearing service at the weekend and on your well-earned holiday to the Maldives. Customers are helped, but you are not bothered. And your private mobile number will continue to work as usual. Always, also on holiday! (Pssst... you can forward your mother-in-law directly to voicemail - always!