When I insert a verizon SIM with a contract or prepaid, the device works perfectly, but the website says it is not compatible with the network. Specifically, "The phone associated with the Device ID you entered is not compatible with the Verizon Wireless network."
If you didn't buy the phone from Verizon then the IMEI number is not in Verizon's database.
There are only 2 types of iPhone 6 recognized by the Verizon network. These are:
1) Verizon branded iPhone 6
2) SIM free iPhone 6 bought directly from Apple
All other unlocked iPhone 6 will not be in Verizon's database. If you put an already active SIM in the phone, it will work, but the network will report it as an incompatible or unknown phone. It also will not be eligible for the $15 or $25 per month out of contract (month 2 month) access pricing.
Yeah, obviously. But WHY? My phone is unlocked, why won't Verizon recognize it as a valid device? It doesn't make sense if other unlocked phones from Apple work.
Verizon is still at its base a CDMA network that is not based on SIM cards but registered IMEI number and Verizon chooses to not add these. Verizon could if it wanted too, but for whatever business decision is does not. Obviously.