Overview
This article explains how SMS delivery works, why messages may occasionally not reach recipients, and what redundancy measures are in place within our communications service.
While SMS is a highly effective channel, it operates on a best-effort delivery model and cannot guarantee 100% delivery to end devices.
Main Points:
- SMS delivery cannot be guaranteed to the handset.
- We maintain logs up to the destination carrier handoff.
- Carriers and devices apply independent filtering.
- High-volume events may temporarily affect deliverability.
- Email redundancy is used to mitigate risk.
SMS Delivery Path
When a message is sent, it travels through several systems before reaching the recipient:
- Our platform
- SMS gateway / aggregator
- Destination carrier network
- Recipient’s local mobile carrier
- Recipient’s device
We maintain delivery logging up to the point where the message is successfully handed off to the destination carrier network. Beyond this point, delivery depends on external operators and handset conditions, which are outside our control and visibility.
Why SMS Messages May Not Be Delivered
1) Device-Level Spam Filtering
Modern smartphones apply spam detection to SMS messages.
Examples include:
- AI/ML filtering on Android devices (e.g., Google Messages)
- Manufacturer filters (Samsung, Huawei, etc.)
- Apple iOS sender reputation filtering
- User-configured blocking or keyword filters
Messages may be blocked, hidden, or redirected to spam folders without carrier feedback.
2) Carrier Filtering & Firewalls
Mobile carriers operate spam and fraud protection systems that scan incoming traffic.
Messages may be filtered or blocked due to:
- Suspected spam patterns
- High-volume traffic bursts
- Complaint or opt-out rates
- Suspicious URLs or domains
- Sender ID reputation
Carrier filtering can occur silently, without delivery failure notifications.
3) High-Volume Event Traffic
Large communication spikes (e.g., events, alerts, bulk notifications) can trigger automated carrier protections.
In such cases, carriers may:
- Throttle delivery speeds
- Temporarily block a sender ID
- Filter traffic for review
Delivery typically normalizes once traffic levels return to standard patterns or sender reputation stabilizes.
4) Sender ID Recognition
Alphanumeric sender IDs (e.g., “BM Info”) build reputation over time.
Filtering risk may increase if:
- The sender is newly introduced
- Traffic volume increases suddenly
- The sender lacks prior carrier trust signals
Adjusting the sender name can sometimes improve deliverability.
5) Handset & Network Conditions
Additional factors include:
- Device storage limitations
- Network outages
- Roaming restrictions
- Inactive phone numbers
- SIM or service suspensions
Delivery Rate Expectations
SMS is not a guaranteed delivery protocol.
Typical enterprise SMS deliverability ranges between 95–99%, depending on:
- Geography
- Carrier policies
- Traffic patterns
- Sender reputation
Because of this, SMS should not be used as the sole channel for critical communications.
Redundancy via Email
To improve communication reliability, we provide email as a secondary channel.
If SMS delivery is impacted, email acts as a fallback. However, email deliverability can also be affected by:
- High-volume sending
- Domain/IP reputation
- Spam filtering
- Provider throttling
We actively monitor and remediate such issues.
Mitigation Measures
If delivery issues exceed acceptable thresholds, we may implement:
- Sender ID changes
- Traffic pacing adjustments
- Carrier route optimization
- Volume throttling
- Template/content changes
Note: Sender IDs are limited to 11 characters.