GST registration contact updates require departmental verification and authorized signatory confirmation before portal changes. Advisory prescribes that SEZ units or business verticals under the same PAN register via the standard New Registration link with distinct mobile numbers and e mail IDs; tax practitioners must avoid simultaneous enrolment windows and clear browser cache to prevent data-mix. It sets a standard operating procedure for changing the authorized signatory's contact details: written request to the nodal officer, departmental verification that contacts belong to the authorized signatory, and forwarding of GSTIN, taxpayer name and new contact details from the department to the specified mailbox for GSTN update.