I cannot activate my iMessage even after all the steps given to me here

First, check to know whether you have an SMS plan enabled at least for One SMS with your carrier. Contact your Carrier (Cellular Service Provider) the agency who issued the SIM to you

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 SMS Only)

See the pic below

4 Step Process --> Turn off and restart iMessage and FaceTime (The order/sequence is very important)

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on (Watch the GIF hereunder)

I found a solution for this. Tried all steps tried resetting tried new software tried inputting phone number in iCloud tried removing sim the. Re-inserting it but same error. Cannot activate.

i found a very simple solution and it is: Activate your iMessage and FaceTime using 3G coverage not 4G

Enjoy

How using 3g?

before activating iMessage

go to settings>celullar>Celullar data options>Voice & Data> and put it 3G only

restart your iphone then activate your iMessage & Facetime