iPhone Call Merge and Call Hold Features Are Disabled
I encountered an odd issue while upgrading from an iPhone 12 to iPhone 15 Pro Max. I successfully transferred all data from my old iPhone 12 to the new iPhone 15 in order to set it up. I also fully wiped the iPhone 12 and traded it in to AT&T for the rebate. Both of this upgrade practices are fairly normal and is what I also did when I received the iPhone 12 year ago... however this time, "merge calls" and the ability to place a call on hold were both disabled in the user interface.
I found similar posts and they were either closed due to inactivity or had a simple cut-and-paste solution with no follow-up regarding whether or not the issue was fixed.