Ring 0345 454 1111 from any phone (or 150 from a VM phone.) I'd suggest going with either Option 4 -> Option 1 (Customer Care Team) from the main menu, or option 4 -> Option 5 (Customer Relations Team) from the main menu between 8AM to 8PM M-F and 8AM to 6PM Sa. They can activate it for you during the call in.
Or make the same request via the Webchat from a desktop or laptop, between 8AM to 8PM M-F and 9AM to 5PM Sa: Account and Billing -> My Broadband, TV and Home phone package -> Chat online. If the agents are all busy, reload the page every 3 minutes and check again, until you get through.
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Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.