Porting your landline number to ONSIM is a smart move, but if you have ADSL or FTTC (Fibre to the Cabinet) broadband, there is a massive "gotcha" that could knock your entire home or office offline.
On traditional UK copper lines, your phone number and your internet connection are "married." If you tell the system to take the phone number away (porting), the system assumes the line is no longer needed and automatically triggers a cancellation of the broadband service.
The "Ghost Cancellation" Risk
When you submit a port request to ONSIM without preparing your broadband provider first:
The Port Completes: Your number moves successfully to ONSIM.
The Line Ceases: Your broadband provider receives an automated "cease" notice.
The Connection Dies: Your internet cuts off immediately.
The Penalty: You may be charged early termination fees by your old provider because the system thinks you just quit your contract without notice.
How to Prevent the Blackout
Before you start the porting process with ONSIM, you must perform a "Broadband Bridge." Follow these steps exactly:
Step 1: Contact your Current ISP. Call your broadband provider’s technical or "retentions" department.
Step 2: Request a "Renumber." Tell them you intend to port your voice number to a VoIP provider but must keep the data service active.
Step 3: Ask for a "Data-Only" Line. Ask them to provide a new, dummy underlying number for the broadband service (often called "renumbering the lead line").
Step 4: Confirm the "Bridge." Ensure they have flagged your account so that the outgoing port of your current number does not trigger a "Cease" order on the broadband.