My phone not recognising one number

What do you mean by that? You dial that number, and nothing happens, or some iPhone-specific feature does not work with that particular phone number?
 
Solution

My Samsung phone on Tesco mobile does not recognise an iPhone number on phone book but contact when the number is dialled
 
I still can't get what is not working... Can you dial that number from your phone, and talk to it - yes or no?

For sure, iPhone-specific features like FaceTime are not going to work.
 


 
By a chance - are you dialing cross-country? May be your phonebook, for that particular person, does not contain the phone number in proper format...