Doro phone can sent text but don't arrive

[Tonymay]

Recently my mother's doro phone has an issue. The phone can received texts but when she sends a text the phone said it has been sent but it never arrived. She has a unlimited text plan. I checked the SMS Centre number, seems correct and I put my SIM in her phone and it's working. So I bought another Doro phone for her but the new one has the same problem. Do you think it could be the SIM itself? 

Any ideas will be very helpful, thanks.

[Peter_W]

Welcome to the Community, @Tonymay 

Is it all numbers that are affected here, or only certain ones?

Could you please let me know what message centre number is showing, too?

Just as another thing to check, has she ever used an iPhone in the past?

If a customer switches from an iPhone to a different kind of handset, they need to de-register their number from iMessage as this can affect their ability to send texts to other iMessage users

You can sort this out here:

Deregister and Turn Off iMessage - Apple Support

Peter