I have an issue sending text message to just one number.

I have an issue sending text message to just one number.

It all started 2 weeks ago when I tried to force an i-cloud backup which failed. I re-booted my phone and when it came back on I had lost all my message histories (I hardly ever delete messages)

I have been able to send and receive text messages and calls from all i-phone and android phones except for 1 android phone - my mums! She can send texts to me, and I can receive and make calls to her. She received texts from other phones.

I have tried:-

Airplane mode on and off.

Switching phone off and back on

Re-booting.

Deleting number from phone and contacts and re-installing

Contacted service providers who were less than helpful but say no issue at their end

I have had an update installed since the issue occurred, and have also been able to back up i-cloud successfully since. My previously disappearing text histories never re-appeared.

Any other suggestions? It's driving me crazy!