The UKVI Employers helpline is Tel: 0300 123 4699
Requests for confirmation of a person’s right to work can be made by completing a request form which can be found here
Once completed the form should be emailed to the Employer Checking by e-mail at
Employerchecking@ukba.gsi.gov.uk
Alternatively, contact the Immigration Enquiry Bureau on 0870 606 7766 or e-mail at
UKBApublicenquiries@ukba.gsi.gov.uk