Well, when you think about the amount of data collection required to reach this level of coverage I don't blame a smallish start-up like Twilio for not having a library.
After all, if you want to test to see if a number is valid, try and call it.
After all, if you want to test to see if a number is valid, try and call it.
The problem is this. Although there are hundreds of websites with UK area code lists, almost all of them are either incorrect, out of date, full of typos, or are generally unreliable. Even lists from "official" sources cannot be relied on with any degree of certainty. The obvious errors are easy to find and correct. It took a long time to find out there were many non-obvious errors and there are likely more errors still to be uncovered.
<nitpick>456</nitpick>