Call Merge and Call Hold Features Are Disabled

I noticed that if I went to "Settings | Apple ID, iCloud, Media & Purchases" (the top-most item in settings which also displays your name), there's a group of account-connected devices. I saw my older iPhone in the list and it displayed the current phone number associated with my active device. I viewed the "Device Info" and clicked the "Remove from Account" button. A warning popped up with a header that read "Contact Carrier to Deactivate SIM Card" and description that stated "To prevent this iPhone from receiving text messages and phone calls, contact your carrier to deactivate your SIM card." I press "ok" and the old device was removed. I processed to make a phone call, clicked "add", selected another number and the "Merge Calls" option was now enabled (and no longer greyed out).

I'm not sure if "contacting the carrier to deactivate SIM Card" is required. The local AT&T store salespeople and AT&T Business Mobility Technical Support weren't much help when I inquired about it.

After resetting, clearing & returning my old iPhone, the device's information was still apparently registered & connected to my Apple account. An Apple support stated that this was normal as they don't auto-delete devices. (I shared this information with my business parter and he checked these settings to discover that he had an outdated "iPhone 6" still connected, but it wasn't causing him any call-related issues.)

STEPS TO CORRECT MERGE CALL & HOLD ISSUES:

1. Open "Settings" app.
2. Navigate to "Apple ID, iCloud, Media & Purchases" (the top-most item in settings which also displays your name)
3. Identify any outdated devices listed on this screen. Click to view details.
4. Screenshot the information (just to be safe) and then click "Remove from Account".
5. Contact Carrier? (Unsure if required or not. Apple recommends making contact, so feel free to call your carrier and explain what you just did.)

I'm posting this because I spent hours interacting with both AT&T & Apple support and neither checked out this option to determine whether it could be the issue or not. In the past, if I had phone issues after upgrading to a new phone, I'd visit the local AT&T store and a representative would blame it on the SIM card and insert a new one and provision it with their system. I've owned an iPhone 3, 4, 6, 8, 10 & 12 in the past. They all used physical SIM cards and the "replacement" method was the approach that AT&T took to fix any connection issue. Newer iPhones use an eSIM and it seems there may be some bugs when migrating services to a new device. While I'm not exactly sure what caused the issue, Apple has requested more information from me and are looking into it. I believe that AT&T hasn't logged any information that I've provided as I've never received any ticket #. Each time I've called, I've had to re-explain everything. I don't believe I've spoken with any AT&T individuals that were qualified to do anything other than follow internal scripts & procedures.

[Edited by Moderator]