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Calling from one UK number when abroad to another UK number should be part of your minutes allowances.
If you check the terms in your contract T&C's that should help clarify matters.
Also theres some good info in costs/travelling-abroad.
Customer support via 191 can also officially clarify.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.