Am posting on behalf of my mother who cannot dial out with her phone. It is an iPhone 7. The only recent change relating to the phone is that the account owner (my father) changed (to my mother), approximately a week ago.
The phone receives incoming calls via 4G no problem. It can make/receive calls via whatsapp/over wifi. Both sending and receiving texts via 4G also works without a hitch.
We've checked things like the mobile network settings, scanned for available networks and see/connect to Virgin manually and automatically, tried switching to 3G only and other network modes and tried with wifi turned off, etc.
Whether indoors or outdoors, when dialling out the phone instantly goes to an engaged tone.
Any help appreciated, thanks.