Thank you for confirming JohnPercival
Can you try a network search to see if the issue resolves the routing issue?
Can you try the sim in another handset to see if anything changes?
Are you calling through the phone book or through a message thread? If a message, try deleting the message and try the number again.
When you dial these numbers are you which format are you using? Does anything change when you use +44 or 07 at the beginning of the number?