SIM900 and Phone Numbers
You don't write a phone number to the SIM900.

Your cellular service provider writes the phone number, along with other account information, to a SIM card.  You connect the SIM card to the device.

You will need a separate phone number for the SIM900 - unless you want to pull the sim card out of your phone every time you want to use it.

Even if you could program a new SIM card to have exactly the same information as the one in your phone, your provider would detect that and be very unhappy with you.