With most GSM networks customers activate a SIM card which will work in any compatible device. Before LTE, US CDMA phones didn't have SIM cards, they were activated by the carrier adding the device's MEID serial number to a database of phones allowed to connect to the network. Operators loved the CDMA activation process because it allowed them to force customers who wanted nicer phones to higher priced plans.
LTE phones, including LTE phones for CDMA networks, do use SIM cards. That means that CDMA operators should be able to activate SIMs rather than phones and the SIMs should be swapable between phones. But things change slowly in telecom. Sprint and until recently Verizon would only activate a SIM in combination with a device serial number and would not activate the SIM with a serial number didn't belong to one of their devices. With Verizon, once a SIM was activated it could be moved to a compatible but not officially allowed device and it would work. Actually Verizon had no choice, the Open Access provisions of it's 700 Mhz Block C LTE spectrum prohibit it from restricting customers from using any compatible device. Sprint, which doesn't have any block C spectrum, locks SIMs to the device they were activated with so they either don't work or don't support voice calls when moved to a different device.
Until recently, even though you put but an active Verizon LTE SIM in a Nexus 6 or 7 or an unlocked iPhone 6 or 6 Plus and it would work, However, Verizon would not activate a SIM unless it was in a Verizon device. That's no longer the case. According to a story in 9TO5Mac, Verizon is now allowing at least some non-Verizon phones to be activated. You can check if your phone is eligible on this Verizon page where you can enter the iMEI or MEID of you unlocked non-Verizon device. The page will then tell you if the phone can be activated on Verizon and which Verizon plans the phone can be used on and the list includes Verizon's $45 and $60 Prepaid Smartphone plans.
So far only the Nexus 6, iPhone 6 and iPad Air have been reported to work but other devices that support Verizon's LTE bands probably do. Likely candidates include the Nexus 7, iPad Mini 2 and Mini 3 and the soon to be released Motorola Moto X Android Pure pictured above. Some U.S. Cellular phones have been reported to work with SIM swapping so it might be possible to activate them on Verizon now.
I was also hoping that these changes might mean that the "flashed" Sprint phones that used to work on PagePlus and other Verizon MVNOs could now be activated on Verizon. However, when I entered the MEID of a flashed Sprint Nexus S 4G that had been used for several years on Page Plus the checker rejected it as "not compatible with the Verizon Wireless network".
One big question for many prepaid users is if this means that Verizon MVNOs like Page Plus, TracFone and Selectel can now activate non-Verizon phones. If you have a non-Verizon phone you would like to try enter it's MEID on Page Plus' New Activation page and let us know if it passes.