Key Takeaways
- A free SMS server path still needs hardening: keys, pairing, battery, and webhook signatures.
- Free on SMS Gateway is 1 device and 300 SMS lifetime — not unlimited carrier airtime.
- Isolate OTP from marketing even on a single handset by refusing promo templates on that lane.
- Treat canaries and spare charged devices as security controls against silent outage.
- You bring the Android phone and operator SMS credit; platform meters devices and volume.
What “free” still exposes
A security hardening checklist for a free SMS server is not a marketing page. Free tiers and DIY servers still hold API keys, paired phones, and OTP traffic. Attackers do not care that your plan costs $0.
On SMS Gateway, Free is 1 device and 300 SMS lifetime. You supply the Android and operator credit. Overview: how an Android SMS gateway works. Pricing: devices and SMS volume.
“Free is a budget choice, not a security exemption. A paired phone with SMS permission is still a privileged actor on your account.”
Hardening pulse
Gate
Keys
Gate
Pair
Gate
Webhook
Gate
Canary
Go / no-go matrix
| Gate | GO | NO-GO |
|---|---|---|
| API keys | Server env only; rotated on staff change | Keys in git, chat, or mobile apps |
| Device pairing | Known handset; QR not shared publicly | Unattended shared tablets with admin login |
| Battery / OEM | Exemptions verified after reboot | Silent kill overnight |
| Webhooks | Signature verified; TLS endpoint | Unsigned receiver mutating OTP state |
| Logs | OTP bodies redacted | Full codes in shared Slack |
| Allowance | Aware of 1 device / 300 SMS lifetime | Expecting unlimited free cloud SMS |
Keys and sessions
Keep Bearer tokens in environment variables. Never paste keys into tickets. Revoke on contractor offboarding. Live request shapes stay in Developer Center.
Device and SIM
Follow setup: SMS permission, battery exemption, funded SIM. Free allows one device — that single radio is a single point of failure. Budget a spare for paid plans when you leave Free.
Webhooks and logs
Verify signatures before updating verification rows. Idempotent handlers. See webhook integration. Redact OTP in application logs.
Free plan limits
300 SMS lifetime includes canaries and retries. Sending pauses when the allowance is exhausted — upgrade or custom, not silent aggregator-style overage. Operator airtime is separate and never refunded by us.
Checklist
- Keys in env; no client-side exposure.
- Device paired; OEM exemptions set.
- Webhook signatures verified.
- OTP templates isolated from promo.
- Staff canary passed; 300-SMS budget understood.
Next steps
Related product pages
Jump to the live product docs for this topic—not another long-form article.
- SMS API documentationLive endpoint reference
- device and SMS volume pricingPlans and allowances
- Security and Trust CenterCompliance and posture
- Android SMS gateway product guideDefinition, product, and how to buy
