The FCC allows you to keep and transfer your phone number to any telcom provider. Initiate the number port with the new carrier, and they will handle the transaction with the carrier you wish to dump. Make sure you don't call up Verizon to cancel because that will releases your number and send it back into Verizon's pool.
With you still on contract, I'm not sure if you would need to wait a certain number of days for Verizon to clear your phone ESN before being able to activate it on Page Plus Cellular (should you wish to keep using your current phone and utilizing Verizon's network via leasing agreement with Page Plus).
Much more helpful advice and answers can be found at Hofo's Page Plus forum:
http://www.howardforums.com/forumdisplay.php?f=364
With that said, Verizon is a CDMA network, so there is no SIM chip involved that would allow you to physically swap with TMO or ATT (which are both GSM) and keep the same phone. Different tech, but you can keep your phone number in any case.
The choice then comes down to either TMO or ATT for prepaid if want to play with the actual company and not deal with an entity that leases their networks.
I personally love TMO. It is hands down the best value that includes access to mobile CNN, ABCNews and ESPN without paying extra. Some complain about the limited nature, but you'd actually have to pay for it if you were on their postpaid account.
GSM technology does have a downside in that it can't reach inside buildings as far as CDMA can. TMO mainly operates on 1900 frequency and you should check here for your street level coverage to make sure you'd be getting good reception:
http://www.t-mobile.com/coverage/prepaid.aspx
If you are happy with that, then by all means go for TMO prepaid. The Samsung Stripe just came back in stock at their online store for $29.99. It's a pretty solid compact flip phone with camera, bluetooth, and a nice bright screen. Reminds me of a small bar of soap when closed. Ten 10 minutes are included with activation if you want to test it out before ending it with Verizon. You may still be able to port in your Verizon number later. Of course you can always wait for the free $25 bonus card to come back to make it an even bigger bargain. No guarantee that the phone will be available then though. It's a gamble.
Samsung Stripe:
http://www.t-mobile.com/shop/phones...d&device=ce5ec420-6656-4ea2-a966-38dd7945c5a3
Nothing else in their selection is worth it since the candy bars available are severely lacking in features and none other than the ugly Nokia and Samsung Stripe have bluetooth.
More info on TMO prepaid on HoFo here:
http://www.howardforums.com/forumdisplay.php?f=343
ATT Go Phone prepaid is the other major carrier option. It's decent too, and the main difference with TMO is they operate on both 850 and 1900 frequency -- which can mean better building penetration. Bad side is the higher cost and the annoying notification at the end of EVERY call to tell you how much was spent. Can't turn that message off and it bugs a lot of people. ATT doesn't care. You do have data access on a pay-per-use basis with their various packages, but you already said it's not something you'd use anyway. Any accidental key presses to their data network means pennies per byte out of your account.
HoFo's ATT GoPhone prepaid forum:
http://www.howardforums.com/forumdisplay.php?daysprune=30&f=344
Read up on all your options and go to each carrier's website to find out about coverage in your local area. It's not going to do any good to pick a prepaid carrier that won't have reception where you would make calls.