So you're saying they're a commodity offering with no customer lock-in?
Communication companies like these are inherently highly competitive. Each one only cover some part of the world (for instance, SMS to USA and canada).
When you have a big company operating in multiple continents, like Uber, you MUST have multiple service providers to cover all your customers (and it will also come with costs savings so you really have maximum incentive to diversify).
Once you have diversified to 3+ providers, you mastered the abstraction good enough to plug them in and out. The next logical step is to take advantage of that to put providers in a same zone in competition with each other.
And the interchangeable nature of their business is pretty much the definition of a commodity.
Until then they are the first choice for a large portion of the developer population.
After that, it's just a numbers game: will it cost more to redevelop the functionality they provide (either on your own or with another vendor) than you will save over a reasonable timeframe? If the answer is yes, you keep paying twilio, otherwise out migrate.
I have no first hand experience on what size makes sense for twilio, but this article gives us an upper bound (a huge one, to be sure).