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There used to be ways of diverting a call to get a UK ringtone but that was messy and it may not even work now.
So short answer is you can't personally alter how a person hears the dial tone.
You could always divert straight to voicemail and then return the call if your not wanting a person to know your not in the UK.
Just a thought.
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.