I spent a lot of time playing cat and mouse with this type of toll fraud in 2022.
1. Rate limited SMS by number/ip: bypassed by large number of proxies/vpn.
2. Added captcha: bypassed by attacker manually signing up thousands of accounts (mechanical turks?) over months and then iterating over them for login OTP.
3. Identifying what carriers/operators are involved and blocking them asap (usually obscure ones).
4. Careful monitoring of SMS send rates and alerting of anomalies to investigate.
1. Rate limited SMS by number/ip: bypassed by large number of proxies/vpn.
2. Added captcha: bypassed by attacker manually signing up thousands of accounts (mechanical turks?) over months and then iterating over them for login OTP.
3. Identifying what carriers/operators are involved and blocking them asap (usually obscure ones).
4. Careful monitoring of SMS send rates and alerting of anomalies to investigate.