How to implement MFA using SMS for low cost via Amazon AWS SES service?

Hi,
We are building a SaaS product. We would like to provide MFA via email & SMS to our customers. Is there a reference on how github team implemented their MFA , especially SMS TOP? When we checked on AWS SES MFA via SMS , the price is high. Is there a cheaper way or free method to implement SMS T-OTP? Please provide some guidance.