Here’s how Apple describes the problem:When you update to iOS 26, you might not be able to activate iMessage with your phone number if you have an inactive SIM with the same phone number as your active SIM. Apple says the problem is due to users having an active eSIM with the same number as your active SIM. If the inactive SIM is a physical SIM, remove the physical SIM. If the inactive SIM is an eSIM, tap Delete eSIM to remove the inactive eSIM. Tap Messages.