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.
