1. Check the provider's delivery status first

Every SMS API returns a status callback (queued, sent, delivered, undelivered, failed). "Sent" only means it left the provider. Look for carrier error codes before touching your app.

2. Number formatting

Always store and send numbers in E.164 (+60123456789). Leading zeros and missing country codes are the most common silent failure.

3. Sender ID rules per country

Some countries require pre-registered alphanumeric sender IDs, some block them entirely, and some rewrite them. A message that works in the US can be filtered in India or Indonesia.

4. Content filtering

Carriers filter messages with URL shorteners, certain keywords, or too many links. Keep OTP templates short: the code, the app name, and the expiry.

5. Rate limits and duplicate suppression

Rapid resends can be throttled by your provider or by the carrier. Add a resend cooldown and show it in the UI.

6. Unicode length

A template with emoji or non-Latin characters switches to UCS-2, cutting the limit to 70 characters per segment. Multipart messages fail more often.

7. Test with real numbers in the target country

Mock numbers won't reveal carrier filtering. Test on a real number in each country you launch in. A local SIM works, or an online virtual number service.

Disclosure: I run SMSPing, an online SMS receiving service. You pick an app, country and carrier, get a virtual number, and read incoming SMS in the dashboard or via API. If no SMS arrives, the fee returns to your balance. That makes it a cheap way to check real delivery from abroad.

What's the weirdest SMS delivery bug you've hit?