> There's two languages cell phones "speak" CDMA or GSM. ATT and Verizon use these different wavelengths. To put it very simply think the difference between gasoline and diesel. Yes they fundamentally do the same thing but they're different.

Hello! How are things going for you back in 2009? Weathering the Great Recession okay?

What you said was essentially correct—before LTE became widespread in the United States. Apple was the one of the last major handset manufacturers to put LTE on its devices, which it did starting with the iPhone 5 in 2012. The pre-LTE CDMA/GSM radios that the carriers have on their towers are being shut down as quickly as the carriers can get these legacy devices off their network. These days, if you're not getting either LTE or 5G (at any frequency band) but are instead only getting either EV-DO, HSPA+, or any of the pre-LTE technologies, you're at the very fringes, and as soon as the carriers are able to shut down that radio you're getting nothing. The differences among the carriers is no longer CDMA vs. GSM. AT&T, Verizon, T-Mobile, and USCC all use both LTE and 5G.

Are there differences in coverage among the four major carriers that operate in Maine? There sure are. Are they related to differences between GSM and CDMA? They are not. They are related to different carriers owning different sets of spectrum in different areas and these different frequency bands having different propagation characteristics. T-Mobile's LTE in the 600MHz band is going to provide different coverage from their LTE in the PCS band.

>What you're likely thinking of. Contracts between carriers and pre paid carriers.

No, he's likely thinking about all of the towers that have radios for multiple providers at a variety of frequencies at different heights. It costs a lot of money to build and maintain a tower, so it makes sense that AT&T, USCC, and T-Mobile are going to want to rent space for their own radios on a Verizon tower—and vice versa. This is an entirely separate arrangement from MVNOs.

>If I'm Verizon I have all of this extra "space" on my network. I can avoid heat with the fcc by leasing this to pre paid carriers to let them have lower priority, lower speed service that may receive less high quality voice.

Carriers do sell excess capacity to MVNOs, but they don't do it to "avoid heat with the FCC." They do it to make more money from excess capacity that would otherwise not make them any money.