UK number requiring top-up to call

Hi all
Has anyone found when they call certain UK mobile or landline numbers the call is denied with a message telling me I need to top-up to call that number? I have a £5/month rolling plan that (should) includes unlimited calls to UK mobiles and landlines so why don’t some work?

Thanks very much

This does sound a bit odd MalcolmB5. Calls to numbers charged at a higher rate are chargeable but regular UK mobile and landline numbers should be included. Was it a person or business that you were trying to call?

Hi Martin
Just a family landline and mobile