--- title: SMS Best Practices FAQ | API Docs description: Carrier-level limitations and best practices for SMS delivery via the Linq Partner API V2. --- ## Overview While the Linq API supports sending messages via iMessage, RCS, and SMS, the SMS channel has unique carrier-level limitations and filtering mechanisms that require special consideration. This guide outlines best practices for SMS messaging to ensure optimal delivery rates. ### Important: SMS vs. iMessage/RCS **SMS is fundamentally different from iMessage and RCS.** While iMessage and RCS support rich media, long messages, and high-volume sending, SMS is subject to strict carrier-level filtering and throttling. For best results: - **Use iMessage or RCS for:** Long-form content, media-rich messages, high-volume sending, digest/newsletter formats - **Use SMS for:** Short, simple, human-like messages that mimic natural conversation patterns ## Carrier-Level Filtering SMS messages pass through wireless carrier networks that employ sophisticated anti-spam systems. These systems: - Work on pattern recognition and risk scoring - Cause silent failures (messages appear sent but never deliver) - Are invisible to both Linq and your systems (no error reporting) - Use progressive throttling (once flagged, issues compound over time) ## Volume Limitations ### Message Volume Thresholds Carrier filtering is triggered by high-volume sending patterns: - **10,000-16,000 messages within 5 days** has been observed to trigger carrier throttling - Once throttling begins, delivery rates decline progressively - Recovery requires reducing volume and changing sending patterns ### Message Segmentation SMS messages are limited to 160 characters per segment. Longer messages are automatically split: - **160 characters or less:** 1 segment (optimal) - **161-320 characters:** 2 segments - **321-480 characters:** 3 segments - And so on… **Impact of long messages:** - Each segment counts toward your send rate - A 960-character message = 6 segments = 6x the throttling risk - Messages over 6-10 segments (\~1,600 characters) have extremely high failure rates **Special character encoding:** - Emoji and special characters force UCS-2 encoding - UCS-2 encoding reduces segment size to 70 characters per segment - This effectively doubles your segment count for messages with special characters ### Recommendation Keep SMS messages under 160 characters whenever possible. For longer content, use iMessage or RCS instead. ## Content Best Practices ### URLs and Links Carriers actively filter messages containing certain link patterns: 1. **Multiple URLs in a single message** - Frequently silently blocked by carriers - *Solution:* Limit to one URL per message, or use iMessage/RCS 2. **Link shorteners and tracking URLs** - Carriers flag URL shorteners (bit.ly, tinyurl, etc.) as potential threats - Tracking parameters can trigger spam filters - *Solution:* Use direct, untracked URLs, or use iMessage/RCS for tracked links 3. **Suspicious domains** - New or unestablished domains may be flagged - *Solution:* Use established, recognizable domain names ### Message Formatting Carriers interpret certain formatting patterns as automated/bulk messaging: **Avoid in SMS:** - Bullet points and numbered lists - Multiple paragraphs - Digest or newsletter structures - Highly formatted content - Excessive punctuation or ALL CAPS **Prefer in SMS:** - Single paragraph, conversational text - Natural sentence structure - Human-like tone and pacing - Simple, plain-text formatting **For formatted content:** Use iMessage or RCS instead. ### Special Characters and Emoji - Emoji and special characters force UCS-2 encoding - UCS-2 reduces message capacity from 160 to 70 characters per segment - This doubles your segment count and accelerates throttling - **Recommendation:** Avoid emoji in high-volume SMS. Use iMessage/RCS for emoji-rich messages. ## Media and Attachments ### MMS (Multimedia Messaging) Sending images and media via SMS/MMS has additional limitations: 1. **Faster throttling** - MMS triggers carrier limits faster than plain SMS - Multiple images accelerate rate limiting 2. **Delivery latency** - MMS messages deliver more slowly than SMS - Bandwidth constraints on carrier networks 3. **Out-of-order delivery** - MMS messages may arrive out of sequence - Especially problematic when mixing SMS and MMS in the same conversation ### Recommendation For image and media sending, strongly prefer iMessage or RCS. Only use MMS for occasional, low-frequency media sharing. ## Send Pacing ### Human-Like Patterns Carriers detect and filter automated sending patterns. To maintain deliverability: 1. **Implement send throttling** - Space out messages to mimic human conversation patterns - Avoid burst sending (many messages at once) - Consider implementing a queue system 2. **Vary send timing** - Don’t send messages at perfectly regular intervals - Avoid sending large batches at the same time of day 3. **Monitor volume** - Track your daily and weekly SMS volume - If approaching 10,000+ messages in a 5-day period, consider switching to iMessage/RCS for some recipients ## Recovery from Throttling If a phone number becomes flagged by carrier filtering: 1. **Reduce volume immediately** - Stop or significantly reduce SMS sending from that number 2. **Change content patterns** - Avoid the content types that triggered filtering 3. **Allow recovery time** - Carrier filters may take time to reset 4. **Contact Linq support** - We can work with carriers to investigate, but recovery is not guaranteed Prevention is critical - Once flagged, a number may remain throttled even after reducing volume. ## Best practices summary ### Do - Keep messages under 160 characters. - Use single, direct URLs (no shorteners). - Write in a natural, conversational tone. - Implement send pacing/throttling. - Monitor your sending volume. - Use iMessage/RCS for rich content, media, and high-volume sending. ### Don’t - Send high-volume SMS (prefer iMessage/RCS for scale). - Use multiple URLs in one message. - Use URL shorteners or tracking links. - Use bullet points, lists, or newsletter formatting. - Send long messages (over 160 characters). - Send frequent MMS/images via SMS. - Use emoji or special characters in high-volume SMS. - Send in burst patterns. ## When in Doubt Default to iMessage or RCS for any messaging that involves: - High volume (hundreds to thousands of messages) - Long messages or rich formatting - Multiple links or media attachments - Marketing or newsletter content - Time-sensitive delivery requirements SMS should be reserved for short, simple, conversational messages sent at human-like volumes and pacing. ## Support If you’re experiencing SMS delivery issues or have questions about your specific use case, contact your Linq representative or email .