Considering it does NOT look to support inbound or voice probably very little impact. (edit) i some how always forget the most important NOT operators.
Low impact in the short term, potentially huge impact in medium term (1-2 years). The pattern for other AWS services is that they will drive down price until they are low price provider and quickly close feature gaps. Adding inbound to SMS seems like a natural next step.
Also, SMS is a substantial part of Twilio's business. Even though Twilio has been adding new extensions to their product, the majority of usage is tied to a small set of API calls.
Twilio now has a company with a huge developer audience going straight after their core business. One that is willing to be very aggressive on price to win share.
Yes -- this is why it seems risky to invest in Twilio long-term.
Sure, they have the developers today, but if WhatsApp leaves, and Amazon can cross-sell more of their existing customers what they'd otherwise get from Twilio, that's a big problem long-term. Twilio has a bit of a moat with their carrier/provider relationships, but that won't hold up against Amazon's entry.
Time will tell, I guess. The stock has been going crazy the past few days, though