I ran the engineering side of carrier integrations at WhatsApp. Carriers wanted to sell data plans with special pricing for data with WA and use WA branding in advertising, because it attracted customers that might later convert to a bigger general purpose data plan. As part of that, we would ask for zero rated SMS to their customers for verification. When it was available, it was generally faster and higher success vs sending messages through an aggregator.
We also had some, usually small, carriers approach us asking us to set up direct routes to them for verification, because their customers would not always receive our messages when we sent through an aggregator. Early in my career at WA, we would just send these carriers to our aggregator contacts, and often things would get linked up and then we'd still pay $/message but it would work better. As we got a little bigger and built support for direct routes anyway, it was usually not too hard to set up a direct connection and then there'd be no cost for that carrier. Messing around with IPSEC VPNs and SMPP isn't fun and the GSMA SOAP messaging APIs are way worse, but once you get the first couple implementations done, it becomes cookie cutter (and FB had built way better tools for this, and a 24/7 support team, so I never had to be up, on the phone with telco peeps at 3 am kicking racoon or whatever ipsec daemon we were running until it finally connected)
In general, pricing varies widely by destination (country and sometimes carrier), US and some other places are < $0.01, up to $0.10/message isn't uncommon, and some places are $0.20-$0.30/message. Voice calling was usually mor expensive (Twilio should have a price list somewhere for that too; if you can get 6 or 1 second billing, assume a voice verification call is about 30 seconds, but you might have to pay for a whole minute even if you don't use a whole minute).
Those SMTP -> SMS gateways sometimes work in the US, but they don't work much in other countries, and they're not good enough to rely on if your product requires an SMS during the new user flow. SMS costs are real and it's frustrating, but if it costs too much, you need to use something other than phone numbers for ids; I don't think skirting by with email gateways is going to work. But, if you build dynamic routing, I guess you could try.
Also, you've got the use the right email gateway for the user's carrier, and a carrier lookup is on the order of $0.01, unless you have tons of volume, so for the US, you might as well pay for the SMS.
[1] https://assets.cdn.prod.twilio.com/pricing-csv/SMSPricing.cs...