They won't activate ANY device that doesn't have a Verizon or Sprint IMEI and wasn't sold originally as a Verizon or Sprint device. It's because they have to use CDMA for voice and fallback data which is NOT part of the LTE network the SIM connects to. There is no such thing as an unlocked CDMA phone. Until VoLTE, you will not be able to use an unlocked phone on Verizon, Sprint, U.S. Cellular, etc. Even then there will be SIGNIFICANT disadvantages versus using one with CDMA fall-back because you MUST have an LTE signal to do anything.
That's why the Verizon version of the latest iPhone is always better than the SIM unlocked one that is never available at launch (iPhone 6 SIM free unlocked phone just became available less than a day ago). It is unlocked for any GSM carrier (AT&T, T-Mobile, international carriers) AND it can also be used on Verizon.
From Apple's iPhone 6 Store page.
I guess Apple was able to convince Verizon and Sprint to add all unlocked iPhone ESNs to their database. Wasn't Google able to convince both Verizon and Sprint to do the same with the Nexus 6?
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