The primary identifier used in any accounting system to track supplier invoices is the invoice number. Since any accounting software package will immediately flag an invoice if that invoice number has already been entered, having a unique invoice number for every invoice is a significant control issue.
This is a real problem when receiving invoices from utilities or telephone companies, as well as employee expense reports, since they don't usually contain an invoice number. The problem is exacerbated when the supplier customarily sends multiple invoices for the same period of time.
The basic principle for fixing this issue is to use other information on an invoice -- that is unique to that invoice -- to derive an invoice number.
The most common work-around to this problem is to create an invoice number that matches the invoice date. For example, if the cell phone company sends a bill that is dated January 15, 2010, the invoice number can be 011510. However, what if the phone company sends multiple invoices that were all created on the same date? This is extremely common, since many large companies print invoices in batches on the same date. The work-around becomes much less palatable, for the assigned invoice number now becomes essentially meaningless. For example, if three invoices are received from the same phone company, and all three have the same date of January 15, 2010, then their invoice numbers become 011510a, 011510b, and 011510c -- hardly a good way to assign a unique invoice number to an invoice!
Other alternatives can yield better results. For example, what if an employee prepares several expense reports, but forgets to submit them for some time, until he suddenly inundates the accounting department with five of them on the same day (a common scenario with traveling salespeople)? The best way to create a unique number for each of these expense reports is to use the date range of travel dates shown on each invoice. For example, if the earliest and last dates of expenditure on an expense report are January 12, 2010 and January 15, 2010, then use 011210-011510 as the invoice number. It is highly unlikely that an employee will submit more than one invoice having the same set of date ranges, so this approach works well. This is also a useful technique when reviewing expense reports on a vendor ledger, since all expense date ranges are immediately visible just by looking at the invoice numbers. However, be sure to use exactly the same format every time when entering this invoice number, or else the system will not flag duplicate invoice numbers.
But what about those pesky cell phone bills? Not only are multiple phone invoices usually printed on the same date, but they also cover the same date range, making the last solution unusable. Instead, it might be possible to incorporate into the invoice number both the date range and the area code for the phones listed on the invoice. For example, if the phones listed on the invoice are for the 501 area code and the invoice is for January 2010, then the invoice number could be 501-012010.
Another alternative is to have the supplier combine multiple invoices into one, though this is not likely if the invoices are being generated by different billing systems. A better alternative is to request a different service period from the supplier for some services, which should result in invoices being printed on different days, thereby allowing for the use of date-range invoice numbers.
In short, the invoice numbering scheme used will vary for each supplier, depending on the unique identification features found within each invoice, the date ranges used, and the number of invoices received.