The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

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Tuesday, April 23, 2019

Dealing with phone numbers

“Project: Wolowizard only supports NANP numbers, but since those numbers come via The Protocol Stack From Hell clearly marked as NANP, it's easy to determine there if a number is NANP or not. It's not quite as simple in “Project: Sippy-Cup” since SIP is … a bit loose with the data formatting.

There, the numbers are formatted as a tel: URI (or a sip: URI but the differences are minor). If the number is “global,” it's easy to determine a NANP number because it will be marked with a “+1” (“1” being the country code for North America). So, tel:+1-501-555-1212 is most definitely a NANP number, while tel:+501-555-1212 is not.

Things get a bit more muddy when we receive a so-called “local” number. RFC-3966 clearly states that a “local” tel: URI MUST (as defined in RFC-2119) contain a phone-context attribute—except when it doesn't (I swear—the RFC contradicts itself on that point; tel:8005551212 is valid, even though it's a “local” number and missing a phone-context attribute because it's a “national freephone number”). So tel:555-1212;phone-context=+1501 is NANP, while tel:555-1212;phone-context=+501 is not (look closely at the two—one has a country code of “1” while the other has a country code of “501”). It's worse though, because while tel:555-1212;phone-context=+1501 is NANP, you cannot use the phone-context attribute to reconstruct a global number (the RFC contains the following example: tel:863-1234;phone-context=+1-914-555—um … yeah).

To further complicate things, the phone-context attribute does not have to contain digits—it can be a domain name. So tel:555-1212;phone-context=example.com is a valid number. Is it NANP? International? Who knows?

So what does “Project: Sippy-Cup” do? If it receives a “local” number with a “+1” country code in the phone-context attribute, it's marked as NANP; any other country code is it marked as non-NANP. If the phone-context attribute contains a domain name, it is treated as a NANP number (based on what I saw in production). And if there's a missing phone-context attribute for a “local” number, “Project: Sippy-Cup” treats it as a NANP number if it has at least 10 digits.

Now, why do I care about this? Because we want to avoid doing an expensive database query for non-NANP and invalid NANP numbers, but “Project: Heimdall” wants all the numbers for tracking potentially fraudulent calls.

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