Reliable SMS Delivery
What makes SMS arrive — and what to do about the cases where it does not.
What "Reliable" Actually Means
Every SMS provider claims high delivery rates. The number that matters is not the one on the marketing page — it is whether your OTP arrives in four seconds at 11pm on a public holiday, and whether you can tell what happened when it does not.
Three things decide that: the route your message takes, what happens when a route fails, and how honestly the result is reported back to you. This page covers all three, including the parts that are outside anyone's control.
1. Route Quality
Why number portability matters. When a subscriber moves their number to a different operator, the prefix no longer tells you which network they are on. A gateway that routes on prefix alone sends to the wrong network and the message fails. iSMS resolves the current network before routing. More on MNP support →
2. Redundancy — What Happens When Something Breaks
The SMS API is published on four hosts. www.isms.com.my is the production endpoint; three mirrors exist purely so your integration can fail over automatically if the primary is unreachable. The platform itself runs with a live failover environment on a mirrored database.
3. Honest Delivery Reporting
This is where most providers are vague. In Malaysia, a Delivered status confirms the telco's SMSC accepted your message — it does not confirm the message reached the handset. Malaysian operators do not provide handset-level confirmation on standard routes, and no provider can give you what the operator does not return.
We say so plainly because building business logic on a misunderstood status causes real problems — for instance treating an API 2000 response as proof a customer received their OTP.
- Delivery reports are available on two-way SMS and long/random number routes
- They are not returned for registered alphanumeric Sender ID routes in Malaysia — operators do not send receipts on those
- If you need confirmed Sent → Delivered → Read at the handset, that is WhatsApp Business API, not SMS
Full DLR status reference → | Reading your delivery report →
Why Messages Fail — and What to Do
| What happened | Status you will see | What to do |
|---|---|---|
| Handset off or out of coverage past the validity period | UNDELIVERED | Nothing — the SMSC already retried. Resend later if the message still matters. |
| Number inactive, disconnected or never issued | UNDELIVERED | Flag it in your database and stop sending to it. |
| Recipient inbox full | UNDELIVERED | Out of your control. Common on older handsets. |
| Wrong number format — missing country code, spaces, dashes | -1008 | Normalise to international format with no +, e.g. 60123456789. |
| Destination network unreachable or blocked to the carrier | FAIL | Nothing to do — Fail messages are not charged. |
| Message content filtered | -1009 | Check for prohibited content. URLs in SMS are not supported for Malaysian destinations. |
| Request came from an unapproved server | -1003 | Whitelist your sending IP with support before going live. |
Improving Your Own Delivery Rate
A meaningful share of failed messages are fixable on the sender's side. In rough order of impact:
- Clean your list. Store the status against every number and stop sending to ones that repeatedly come back UNDELIVERED. Stale lists are the single biggest drag on delivery rate.
- Normalise numbers before sending. Country code, no
+, no spaces or dashes. Do it in code, not by asking users to format correctly. - Do not paste from Word. Curly quotes and hidden characters flip your message to Unicode, cutting the limit from 153 characters to 63 and tripling the cost. Character limits →
- Leave no links in Malaysian SMS. URLs are not supported for Malaysian destinations. Use WABA if the message needs a clickable link.
- Register your Sender ID early. Approval takes time and is tied to message content. Sender ID registration →
- Send campaigns outside peak hours. Gateway traffic spikes around festive periods; schedule with headroom rather than sending at 6pm on the eve of a holiday.
Account Security & Control
Coverage
- Malaysia — all operators, direct routes
- Southeast Asia — Singapore, Philippines, Thailand, Vietnam, Cambodia, Brunei, Myanmar, Laos
- Asia Pacific and beyond — China, Hong Kong, Taiwan, Australia, India and 190+ countries
Rates and available networks differ by destination. Search the full coverage list → | See MYR rates →
Related: One-Way Bulk SMS → | Two-Way SMS → | SMS Reports →
Please include your iSMS username in any support email.