You could try AWS SNS SMS. This is also PAYG. A wrapper like https://sendwithses.com might make your work easier.
EDIT: With AWS SNS SMS you can set a spending limit and your bill will never go over that.
EDIT: With AWS SNS SMS you can set a spending limit and your bill will never go over that.
Source: use it and twilio.
Looks like we'd need https://docs.aws.amazon.com/pinpoint/latest/userguide/channe...
Which isn't bad, but is much more limited than what Twilio provides.