I can’t text smartphones after moving to Ireland, can someone please help?

I have recently moved to Ireland with an unlocked iPhone 7. I’m using my new Irish connection in the device and I have changed my iPhone settings so the selected country is Ireland.

I can call anyone in the UK, but when I try to text the same numbers I have dialled it won’t send my messages, unless the end user also has an iPhone meaning we can iMessage. I can even WhatsApp smartphone users, but I can’t send a standard text message.

When I sent a text it will say “message failed to send to (071)********but in the contact page that number is saved as (+44*******. I have dial assist on, but it doesn’t appear to make a difference for text messages.

This must be an issue with the iPhone recognising numbers in a certain format and not realising the international dialling code, but is there a reason why SMS won’t work?

Any advice would be much appreciated. Thank you.