Hey there and welcome to the Samsung UK and EU Community Customer to Customer Forum
Sorry to hear of your issues. As has been said, this is more likely to be a carrier issue. You may simply live in an area or not spot with limited coverage.
Have you tried enabling WiFi calling to see if that resolves the issue for you?
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I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.