Hi ​@DarrenSm 

Please check the below:
Settings > About Phone or About Device > Status > My phone number.  Some Android phones will have SIM or SIM card status listed within Status.  This is how you will be able to find the number you are using on your phone and sim. if using Message App,  Change the phone number associated with the Messages app:  Open the Messages app.  Tap More  then Settings  then Advanced  Select Phone number  then edit the number to the correct phone number associated with your SIM card.  Tap OK.    If still have issue, remove the SIM and reboot the device.  Then re-insert the SIM and check if its updated the correct port number in Phone settings.    Still have the same issue after rebooting  then advise try the SIM once in different device and  see while sending text does it shows temporary number or port number.

Already tried all this and tried them again after reading this post. Still nothing.

And the abysmal customer service continues. After speaking to them yesterday midday and being told I’d be contacted in 48hrs, I decided to call them by phone to try and speed things along - I’m missing very important calls! Explained all the above and was told I'd have to wait another 72hrs! Told them this was unacceptable and wanted to speak to a supervisor/manager, at which point they went silent and hung up on me!

I should also point out I tried to post this a few hours earlier, but apparently their moderators like to censor any signs of complaint, they can’t deal with… Unlucky for them, this whole message is copy/pasted in a notepad and all other social media channels.