who owns my cell phone number

Sorry you lost your old number. Next time if you are inbetween carriers, I would look into something like Google Voice. Some folks have found porting to GV for a one time $20 fee is one useful method until you can sort out your next carrier/handset/etc. See here:

http://www.techrepublic.com/blog/google-in-the-enterprise/take-control-of-your-mobile-number-with-google-voice/

Otherwise, to elaborate on what Bob said (and also spot on, btw), normally you sign up for the second carrier while you are still on the previous carrier. The new carrier provides you with a temporary number, which you test out for the first 30 days (etc.). Once it works to your needs, then you port over (=transfer) the old number to the new service. Any cancellation otherwise means you relinquish your old number.