Got a new Samsung S21 today, upgrade from A40. O2 shop said to swap SIM card over. Cannot make or receive calls, checked with Samsung support, they checked everything and said I need a replacement SIM. O2 chat support is too busy, and obviously I can't phone them!!! Can anyone help? Thoroughly unimpressed so far.
You might try cleaning off the SIM contacts with a dry, lint-free cloth first, and putting it back in the new phone in the correct orientation.
FAiling that, an O2 Shop might be a place to try for a replacement, and failing that, it is going to have to be through one of the numbers for Support, all in this Guide: Coronavirus Community Help and Support, @EJ19.
The best time to actually stand a chance of the call being answered by someone is around 08:15 BST (UK time) in the morning.
Have you tried your old sim card?
There should be no signal on the old sim if the swap has been done
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