Hey there and welcome to the Samsung UK and EU Community Customer to Customer Forum
Sorry to hear of your issues.
When calling from abroad, you need to dial +44 followed by the number of the person you are calling but without the first 0.
If that doesn't work, then you may have to call your network as you may not have Roaming turned on.
Or you may have a spend limit which may be £0 or you may have exceeded it.
Or you may have a tariff that doesn't include Roaming in the EU?
S24 Ultra