Then to keep that number, you will have to port it out.  If under contract, you will have an ETF.  If not under contract, then you can leave whenever.  So, after you port out, you will have your number on Twilio and then you can get a new line and phone with Verizon, or get a second line before you port out to make that easier.  However, doing so will either get you into an Edge payment, 2 year contract, or BYOD to a plan or prepaid.