This number is registered to your apple ID, but is not associated with this phone.

alaazs wrote:

I also tried turning the Facetime and imessage off and restarting the phone but no use.

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)

Just happened to me. Faster to just remove your phone number in iMessage under Settings, (where it says only 6 days left) and then put it back on.

On your iPhone, go to Settings > Messages > Send & Receive. Select your Apple ID at the bottom of the screen and tap Sign Out. Then go to Settings > FaceTime, select your Apple ID and tap Sign Out. If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password.

Add or remove your phone number in Messages or FaceTime ...

alaazs wrote:

It says expiring in 4 days

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images / pictures speak louder than words. Do not provide any personal credentials, such as email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable. Etc.