How can I find out which Gateway User ID was used to submit previous VAT returns?

I called by phone but after 45 min on hold I gave up. We attempted to add VAT to our Tax Account, but encountered an error message. You cannot use this service Your business is set up for VAT with another Goverment Gateway user ID. Try signing in again using a different user ID. Any idea? Thank you in advance.

You'll need to call HMRC to resolve this. Call the VAT IT Online Helpdesk - this helpline only deals with logging on/security type queries and so the wait time is less than the general VAT helpline. https://www.gov.uk/government/organisations/hm-revenue-customs/contact/vat-online-services-helpdesk

Hi SkaLa,
You will need to contact the Helpline so that we can resolve this for you.
We do have a webchat option available which you can use to contact us for this type of query.
Please see below:
VAT: general enquiries
Thank you.