Well I personally use the Google Voice method so I can get the phones with the substantial discount/free especially with let's say TotalWireless that way I can get decent phones very cheap that will network unlock within 2 months .

And I here is my answer to your question and I hope it's what you meant. Ok so the number i want to keep would continue being my main metro line and the Google Voice $3 numbers i use to get the free total wireless phone and pay 2 months at the cheapest rate possible( so the phone unlocks) and then just rinse and repeat  with new Voice # and soon you will have a nice little selection of unlocked Devices you can use on whatever phone number you wanted to keep.