I didn't leave because of coverage issues although I had zero coverage when I was at home so I had to use WiFi calling at home even though there's a cell phone tower about two hundred yards from my house. The reason I left ATT was because of their customer service. Over the period of three or four days I spent around eight hours on the phone with their customer support in India trying to get them to fix their billing errors on my account, They couldn't fix their mistakes and I decided it was time to fire them. No regrets. Consumer Cellular has excellent customer support and coverage at half the cost.

Yes, because internet access will be directed to a wifi connection instead of the cellular connection. But the UA wifi, while connected to the phone, may not have internet access on the ground (depending on the aircraft system).

All I know is I went from 0 bars with ATT to 3 bars with Consumer Cellular at my home the day I switched. When I got my sim card for Consumer Cellular they had two choices of simm cards. One was T-Mobile and I don't remember who the other was. I think I got the T-Mobile sim card.I just remember that one of the sim cards would only work in the US while if I was travelling overseas I would need to go with the other.

Been having that trouble at SFO for at least a year, but not in terminal - just in the plane during boarding. I always assumed it was because too many people in a crammed space trying to use the network. Also on ATT.

I am going to switch to google next month anyways.

This explains a lot! My husband was so frustrated with the inability to call or access Internet while waiting in an UA plane until someone told him to turn off the wifi. Once I heled him forget UA onboard wifi, he is a happy guy. My son turns off his wifi regularly whenever he has problem with cell accessibility. I should tell my son to turn off wifi calling instead. It provides zero benefits with Google as the cell provider.

It's Worse in the Polaris Lounge
 

After returning from Japan a few days ago, I was trying to make a few calls in the Polaris Lounge at EWR and it was almost impossible. My old flip phone is not internet compatible, so it has nothing to do with Wi-Fi enabling on the phone. It's just bad cell coverage.

As noted, the main cause of bad performance is congestion. As for any network, look to Yogi Berra's saying, "nobody goes there anymore, it's too crowded."

the 700mhz bandwidth segment ATT operates in which is effective at penetrating metal/buildings is also very near the recently acquired (2017) frequency reserved for Air-Ground Channels: 32 6.25 kHz-wide channels (16 pairs) reserved for air-ground communications to be used by low-altitude aircraft and ground based stations. https://www.fcc.gov/700-mhz-public-s...wband-spectrum

My guess is that because the spectrum is close to this frequency that they have to turn down their signal near the airport to comply with their spectrum license (so it doesn't interfere with the other band) for that frequency band which is capable of penetrating buildings/metal.