I can think of 3 reasons they want us to buy their phones:
1. They are making money on the sale of the phones.
2. They want to load it with their particular flavors of bloatware.
3. They are just too lazy to test and make a list of compatible phones.
I doubt that there any true technical reasons that you cannot use your own device. Sure, some phones might not work, but others certainly will. For example, I have an unlocked Galaxy S7 (SM-G930U) that supports all US carriers and just about all current radio bands, GSM and LTE. It supports 19 LTE bands including the measely 4 bands indicated for the xfinity mobile S7. I currently use it on Republic Wireless so I know it also supports wifi calling. if they are using that. They can't tell me that this phone wouldn't work.
I am currently paying $20/month for 1Gig of data on Republic. I make heavy use of wifi and limit streaming, updates, etc. over cell so 1 gig is very achievable. I would certainly give xfinity mobile a chance, but not if I have to buy a new phone.
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