Welcome to Vodafone Community
Is this issue happening everywhere you go or in one place.
Can you try your Sim Card in another phone as a process of elimination to rule out the phone or sim card that may be at fault.
Try another Sim in your phone. If necessary any Vodafone Shop can replace the Sim Card via a Sim Swap.
An Apple Store can run a diagnostic in their geniusbar if required.
You could try a reset reset of the phone by holding the home button and power button in until the Apple logo appears and let go.
Current Phone > Samsung Note9 _512Gb > Model: SM-N960 Duos.
Samsung Gear s3 Frontier Watch.