That said, any ideas for what you could use this for? I'm actually interested in building a sample app for it, I'm just trying to think of a sexier use case than localization and spam-detection. Is that as good as it gets?
1) Smarter Routing - quite a few developers have expressed they'd like to know whether or not a phone number is a landline or not before attempting to send a text message. Twilio Lookup will describe whether the number is mobile, landline, or voip so you can choose the appropriate communication to reach the number.
2) Error Checking - While libraries like libphonenumber are useful for determining if a number can possibly exist, there are some conditions where determining if a number does exist is more valuable. This can be used for error checking - if I as a user input a typo in my phone number (e.g. I mistype my personal phone number as '(347) 193-6073' which does not exist instead of '(347) 923-6073')
I also suspect across larger applications carrier info might also contain useful demography. For example, knowing that your users in Montana mostly use Verizon might inform your advertising to surface more CDMA devices in that market.
Ultimately this will probably end up like every other product we release - the killer use case is one we never even considered.
"Some number types don’t accept all communications -- for example, most landline numbers and many VoIP numbers do not accept SMS messages. Identify which kind of communications users can accept, based on their number type. This allows businesses to optimally decide whether to send voice or text communications for $0.005 per lookup" - from that page
When you type a landline number into my app that sends text messages it will fail. So I can choose to pay Twilio to confirm that the number can receive SMS. My users have mistyped their numbers in the past and that can be awkward when some random stranger gets a text from my system in the middle of the night.
That being said, they would charge me half a cent per lookup. I probably won't bother with local numbers, but I think I will use this for international numbers.
Additionally, since Twilio is giving me national format preference data for free, I might as well reformat a user's entered number to match their usual preference as eye candy. Although, I will just use http://www.phoneformat.com/ JavaScript library like I planned since I want to auto-format numbers as they are typed.
If you are making a site like Craigslist and want to enforce phone verification for making new accounts, you can require that users use a cell phone number instead of easily obtainable VoIP numbers. Craigslist implemented something like this in-house.
Another good use is now you can see if the robocall you received originated from a Twilio number. I looked up my Twilio numbers and they show up as "Twilio" under carrier. The page is live, so you can lookup numbers for free. You can then file a ticket with Twilio and make that person's life miserable.
... "name": "Google (Grand Central) BWI" ...