Selectel Wireless notified their dealers today that effective immediately, they can no longer activate non-Verizon phones. Up until now Selectel had been able to activate virtually any Sprint, US Cellular, MetroPCS or other CDMA phone. Non-Verizon phones had to be "flashed" (reprogrammed) in order to actually work on the Verizon network. Selected itself didn't flash phones but many independent mobile dealers and online flashing services did for around $40 and up.
Selectel says the non-Verizon phone ban is a result of a Verizon policy change that will eventually effect all Verizon MVNOs. Page Plus Cellular, a much larger Verizon MVNO, is still accepting non-Verizon phones at the moment. Selectel told their dealers that the ban will hit Page Plus July 29 but there is no confirmation of this from Page Plus.
If you currently have a non-Verizon phone active on Selectel, I believe that it will continue to work as long as you don't do anything to the account that would cause a deactivation like putting a different phone on the account. At least that's what happened when Verizon began blocking their prepaid phones from being activated on MVNOs several years ago. Prepaid phones activated before the ban still work as long as they have been continuously active on the same Page Plus number.
It's kind on ironic that Selectel got hit with the ban on non-Verizon devices first. Back in May, Selectel told its dealers that Verizon would block other MVNOs from activating non-Verizon phones beginning June 24, but that the banned devices would still be allowed to activate on Selectel because it used the postpaid Verizon network.