@darlicious
I would say it like this:
- 5G = NR
- 4G = LTE
- 3G = UMTS or CDMA Ev-DO Rev. A, which are incompatible with each other
- 2G = GSM or CDMA2000, which are incompatible with each other
In the past, networks were either on the GSM path or the CDMA(2000) path. Most operators in North America used CDMA, and most of the rest of the world used GSM. When you switched providers, you always needed a new phone.
There were 3 paths created for 4G, and initially it was not clear which way the industry would go.
- GSM > UMTS > LTE
- CDMA > Ev-DO Rev. A > Ev-DO Rev. C (UMB)
- WiMAX (brand new technology)
Because of the high costs of patent licenses and royalties demanded by Qualcomm, CDMA became increasingly expensive, and operators decided to abandon it, in favour of other options. A few operators started with WiMAX, but most went with LTE. Eventually early WiMAX adopters regretted their decisions and everyone settled on LTE.
In order to support transitions from CDMA to LTE, especially for voice calls, an operator would have to either deploy a brand new parallel technology overlay (i.e. Bell and Telus UMTS), or convince network + phone manufacturers to support inter-technology compatibility (Verizon). Verizon still had all of their voice traffic on CDMA, and with a new LTE network that had limited coverage and no VoLTE, they successfully convinced their vendors to support their hybrid strategy. But the long term goal was always LTE.
