How do I resolve 'number signed in somewhere else' error for text messaging on iPhone?

To activate iMessage and FaceTime using your Phone number

1. First, check to know whether you have an SMS plan enabled for at least one SMS*** with your carrier. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM to you
2. If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One international SMS*** Only)
3. If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** You may not have an SMS plan (Domestic & International) enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc. does not provide SMS services.

You cannot have one phone number actively registered for iMessage on two different Apple IDs at the same time. iMessage ties the phone number to a single Apple ID for sending/receiving Apple-to-Apple messages. When you register a number with a new Apple ID for iMessage, it usually takes over that number’s iMessage association and removes it from the old Apple ID’s iMessage settings

• A given phone number can only be activated for iMessage/FaceTime under one Apple ID account at a time. If you try to activate the same number under a different Apple ID, Apple’s systems will de-link it from the old one and attach it to the new one. 
• You can still use the same number as a trusted number for Apple ID 2-factor authentication on multiple accounts — that’s a separate purpose and doesn’t give iMessage delivery rights.

if the SMS or RCS text messages are reporting errors, check first with your carrier, then with Apple Support.