Phone says 'No Service'
Is the user in a known poor coverage area?
Ask them to try somewhere with good signal.
Is anyone else on Vodafone or O2 having problems in that area?
If so, it must be a signal issue.
Did the drop in service correspond with a SIM swap or tariff change?
If there was a SIM swap, has the user rebooted their handset since it was completed (check the time on the Eserve)?
If there was a tariff change, has the user waited a full 24 hours before rebooting their handset?