• iMessage Can Be Turned Off And On-Toggling iMessage off and on again can resolve a minor issue that is causing an iMessage activation error on your iPhone. To begin, go to Settings and select Messages.To turn off iMessage, tap the switch next to it. To turn iMessage back on, tap the switch once again. When the switch is green, you know it's on.
• Look for an update to your carrier's settings.-Apple and your wireless carrier make carrier settings updates on a regular basis to improve your iPhone's ability to connect to your carrier's network. To check if a carrier settings update is available, open Settings and go to General -> About. If an update is available, a pop-up will show on the screen within a few seconds. If a pop-up window displays, select Update.

Read More: Does WhatsApp have any drawbacks or problems that they had to face in the past or may in the future?