updated the iOS. restarted my phone turning off iMessage and Facetime. Still it shows activation error for iMessage. Is there any other hack for this?

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.

Do you have access to a Mac or a PC?

Try plugging your iPhone, using an official Lightning cable, into a Mac or PC, and open the Finder or iTunes. Then, wait for your iPhone to activate that way.

If you don’t have a Mac or PC, you can use one from a family member or a friend.

Alternatively, you can always visit an Apple Store for more assistance

Also, could you let us know what the error says, please? 🙏