Why the reference number matters
The reference number is the easiest way to reach the official duplicate-bill flow. If you do not have it, many bill-check portals become useless even before the bill itself is opened.
When to use the official search page
If an old bill is missing, the public IESCO reference-number search page is the right place to start. It is a better path than trying random third-party tools that may not actually connect to the right account record.
Reference number vs customer ID
Some public bill portals accept either the 14-digit reference number or the 10-digit customer ID. The practical advice is simple: use whichever official identifier you already have, but confirm the bill details after the lookup opens.