Start with the device
Write down the exact model and regional variant. A familiar phone name can cover hardware with different supported bands or SIM options. Confirm current UK calling support, including voice over 4G, with the network and manufacturer. The SIM fitting physically is only one check.
Then identify the service
Is it a brand-new number, a replacement SIM, a number transfer or a change to an existing arrangement? These are different processes. Follow the instructions for the action intended, keeping access to the old line if it is needed for verification.
Test three functions
Once activation is confirmed, test a call, a text and mobile data independently. A dual-SIM phone may have different defaults for each. Record which function fails before seeking support; “the phone does not work” gives the provider less useful evidence.