For time-sensitive SMS, one solution is to have basically a fast queue and a normal speed queue. Essentially have a phone number (or numbers) that are reserved

For time-sensitive SMS, one solution is to have basically a fast queue and a normal speed queue. Essentially have a phone number (or numbers) that are reserved for the fast SMS and only use it for time-sensitive requests like OTP. Everything else goes through a normal speed phone number. Alternatively, if all your SMS messages are time-sensitive, you'll likely want to use several phone numbers and try to load balance requests so that the queue size for each phone number is always small/at or near 0. There's also an outside chance that your SMS provider may have an option to pay for faster delivery like email providers do. I don't recall seeing that option, but I have to imagine that at least one provider has this.