Bill's cellular provider is CDMA and he's losing the ability to use his Motorola Moto X. Leo says that the good news is that everyone is moving to LTE/VoLTE, which means CDMA will disappear. Even now, when going with Verizon, you can see the phones have GSM SIM cards. But the older models won't work anymore. A newer one will likely work. Leo says it's time for a new carrier and T-Mobile would let him bring almost any phone to them. T-Mobile also has a pay as you go $30/month plan. It includes 100 minutes a month, unlimited text and data.
Leo says that before he uses his phone with a third party MVNO service like Ting, he should go to their website and type in his IMEI to see if they can support his phone. Another option is to try Google Fi and see if he can get an invite.
(Disclaimer: Ting is a sponsor)