You need an SMS provider. One such clickatell.com supplier that I bought in bulk. They have good service and an extensive API. You have to pay for SMS, but like everything else in life, better material costs more. I found them reliable and their phone support is good.
There are several free ones, but they will obviously be very limited, and it will be difficult to find what can be used for commercial products. You should also be careful with user data, free ones can automatically sign your numbers on the market, etc. Therefore, make sure that your users know about this if you want to accept this route.
It’s just a disclaimer that I don’t have an affinity for Clickatell, and there are many more options, but be careful with resellers who provide sub par service at overpriced prices.