The reality is, when a CDMA carrier deploys LTE without deploying UMTS, there are usually compatibility layers between CDMA and LTE such as CSFB and eHRPD for when VoLTE is absent. CDMA was never designed to interoperate with LTE as LTE was built around IMEIs and SIM cards but CDMA was built around burned-in ESNs and PRLs, so LTE support was hacked on.
This is why Sprint and Verizon used whitelists: they literally couldn't accept GSM-only devices because you wouldn't be able to make a phone call (the phone would try UMTS but only the non-supported CDMA2000 can be used to actually call, so in turn no phone calls).
Sprint went further by using the CDMA provisioning system on top of LTE instead of just using SIM cards and 3GPP provisioning like most GSM and CDMA carriers. This was a nightmare for custom ROM users like me as custom ROMs were designed for GSM carriers in mind and Sprint was at best an afterthought.
This is why Sprint and Verizon used whitelists: they literally couldn't accept GSM-only devices because you wouldn't be able to make a phone call (the phone would try UMTS but only the non-supported CDMA2000 can be used to actually call, so in turn no phone calls).
Sprint went further by using the CDMA provisioning system on top of LTE instead of just using SIM cards and 3GPP provisioning like most GSM and CDMA carriers. This was a nightmare for custom ROM users like me as custom ROMs were designed for GSM carriers in mind and Sprint was at best an afterthought.