Help! iPhone doesn’t recognise phone number

I am power of attorney for my sister so deal with a lot of her phone/financial stuff.

She previously had an iPhone 7 with phone number 1. She moved to an area with no signal for her network, she mainly stays in the house and before we knew it months had gone past so the number had been disconnected from the network.

Went to network provider who gave us a new SIM card with new phone number 2. This number worked fine and she occasionally got signal with this. Could phone when needed.

A couple of months after this she got a new iPhone 14, had to set up a whole new account which meant a new number- phone number 3.

I deleted numbers 1 and 2 from all devices (or so I thought). And now we’ve realised we cannot phone her, it just goes straight to voicemail. We can WhatsApp call her fine.

I have tried numerous times to change the phone number on her iCloud to the correct one (number 3) but I am not receiving the messages or phone calls with the OTP so unable to do anything. Very frustrating as if there was an emergency we wouldn’t be able to easily contact her!

Is this an iPhone/Apple problem or the phone provider problem? Any advice would greatly appreciated