How to audit your carrier or supplier invoice

Most businesses that resell something check the total on a supplier invoice, decide it looks about right, and pay it. This is entirely rational: the alternative is checking forty thousand lines by hand, which nobody will do twice.

It is also where money goes. Not usually through anything dishonest — rates get renegotiated and not applied, prefixes get reclassified, systems disagree about which month a call belongs to. This is how to check, and what to expect when you do.

What you need before you start

Two things. Your own record of what happened — rated events, not a summary — and the supplier's invoice in a data format rather than a PDF. Most suppliers will provide a CSV if you ask; if they will not, that is worth noticing in itself.

Expect the CSV to be awkward. Semicolon delimiters, decimal commas, windows-1252 encoding, dates in an ambiguous order, headers in another language. None of that is a problem as long as you confirm what you have read before you compare anything. A misread column produces a confident, wrong answer, which is worse than no answer.

The four categories

Every difference between their invoice and your records falls into one of four buckets. Keeping them apart is the whole method — a single 'mismatch' figure cannot be taken to a supplier, because three quarters of it is usually not a dispute at all.

Matched

Their line and your usage agree. Usually most of the invoice, and worth reporting explicitly: it is what makes the exceptions credible.

Rate differences

The volume agrees; the price does not. This is the category that most often contains real money. Common causes are a renegotiated rate that was never applied, a prefix reclassified into a more expensive band, or a mid-month change nobody was told about.

You are owed the difference, not the whole line. Anyone reporting the entire line value as 'recovered' is flattering the number.

Timing differences

Real usage, billed in a different period. Typically calls that began before midnight on the last day of the month and completed after it. These are not errors and will clear themselves next month. Reporting them as money owed is the fastest way to lose credibility with a supplier.

Missing charges

They have billed for something with no corresponding usage on your side at all. Sometimes a routing error, sometimes a genuine mistake, occasionally neither. Always worth querying, because unlike a rate difference there is nothing to argue about.

How to run the comparison

• Fix the period first. Agree what 'March' means in both systems, in the same timezone, before comparing anything.
• Normalise units. Their seconds may be your minutes; their per-message may be your per-segment.
• Aggregate to the same grain. Compare prefix-by-prefix totals before comparing individual lines — most differences show up at that level and are far easier to interpret.
• Then drill into the prefixes that differ, and only those.
• Categorise every difference before you total anything.

What to do with the result

Query rate differences and missing charges. Do it with the evidence attached — the prefix, the volume, the rate you hold, the rate they billed, and the difference. A supplier presented with a specific, categorised claim tends to resolve it quickly; one presented with 'your invoice looks wrong' tends not to.

Leave timing differences alone, and say in your query that you have excluded them. It signals that you have done the work properly, and it makes the rest of the claim harder to wave away.

Do it every month

The value of an audit is not the one-off recovery. It is that a supplier who knows their invoices are checked bills more carefully, and that a rate change that does not get applied is caught in the month it happens rather than the year it happens.