Consumer Cellular - BYOP is a Lie

I wanted to switch service for 3 existing phones on an active ATT account to Consumer Cellular. Contacted CC and set up an an account. Provided the IMEI's for the 3 phones. The agent assured me the phones would work with CC. Inserted a CC SIM card in one of the phones (Samsung S8, unlocked ATT phone). Text and data worked but could not make or receive calls. After going through several agents, I was told the phone was not compatible with CC and would have to purchase a new phone. What a scam. I was basically lied to.

I tested the SIM in several other unlocked ATT phones (another S8 and S9+). Same symptoms, no voice calls in or out. I did find the SIM worked in an unbranded international version of the S8. My suspicion is CC (or ATT) is blocking unlocked ATT phones on CC's system.