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I don't know if this is a Vodafone problem or an iPhone issue but I have an iPhone X that keeps cutting out my calls. It appears to be when the phone switches between 4G and 3G but also if someone leaves me a voicemail during a call I'm already on, when 121 appears on the screen to deliver it, it cuts out the current call.
Being on WiFi calling doesn't seem to help so now I'm experimenting with switching off mobile data altogether just to get to the end of a call without it cutting out.
On the face of it that sounds like it's the iPhone and how its switching from 4G to 3G on a call.
My advice is to first seek a diagnostic from a Genius Bar at your most local Apple store if you can.
And or as a process of elimination can you try your sim card in another phone ?
If it's the sim card then a Vodafone Highstreet Store can perform a sim swap to replace it Take photo ID.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.