When I was working in this space, all of my providers gave me a pricing feed which was essentially phone number prefix, price (and some text that my system didn't care about, I don't need a name, just a price, thanks). It looks like twilio doesn't offer that publicly for SMS, but you can see their voice price list by clicking "download voice prices (.csv)" on their voice pricing page [1]. The SMS page has a similar feed, but it only gives you price by carrier name, which isn't very helpful --- you'd need to pay to do a carrier lookup before you could use the price list; these lists don't feel complete either anyway, but it's an idea.
[1] https://www.twilio.com/voice/pricing/us