Use 0345 6000 789 from another phone
you may be able to use the web chat option to speak to an advisor starting from here: Help and Support - Contact Us > Mobile > Mobile not working > Chat online.
The advisors are often busy though so you may not get connected straight away.
Or if you have access to another phone, call on 0345 6000 789.
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.