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Android won't kill sideloading after all, but new verification rules will make it harder
(www.techspot.com)
This is a most excellent place for technology news and articles.
For carriers like AT&T, texting & other data are important. Dumb phones won't provide them the data needed. They will absolutely support Linux phones.
No, there are reports of fairphones (which aren't even linux) not being allowed on ATT already.
Some managed to call customer support to get their their specific phone's imei whitelisted, but like... if the person you get doesn't know what you're talking about, you'd have to keep calling until you find someone competent, so good luck on the "Customer Support hell" with long queue and stuff. And the system could change any time, the manual whitelisting feature could get turned off tomorrow. If there are any issues with service, they'te just gonna blame your phone for not being on their approved list of phone models.