SMS rate limits are determined by your 10DLC campaign type and vetting score. These limits are set by carriers, not by AgentPhone, and apply to all A2P (application-to-person) messaging in the US.
If you exceed your rate limit, messages are not dropped. AgentPhone automatically enqueues them and delivers each message as soon as throughput becomes available.
T-Mobile publishes per-day segment caps based on campaign type and trust score. Since T-Mobile typically represents about one-third of US mobile traffic, you can estimate your total daily capacity by tripling the T-Mobile figure.
A segment is a 160-character chunk of a standard SMS (or 70 characters for Unicode). A single message may count as multiple segments if it exceeds these limits.
High volume campaigns go through a third-party vetting process that assigns a trust score. Your score determines which throughput tier you land in. Companies without an established reputation may initially receive the same 2,000 segment T-Mobile cap as low volume campaigns, even on a high volume campaign. The limit increases as your trust score improves.
In addition to daily caps, carriers enforce per-second throughput limits:
When your sending rate or daily volume exceeds carrier limits, AgentPhone queues the excess messages and sends them as capacity frees up. You do not need to implement retry logic on your end. Messages will be delivered in order as throughput becomes available.
For time-sensitive campaigns, plan your send volume against the limits above to ensure messages arrive within your expected window.
Your limits are tied to your 10DLC campaign registration and vetting score. To improve your throughput:
When sending the first outbound SMS to a contact, your message must include:
Messages missing these elements may be silently filtered by carriers. The API will not return an error — the message will appear sent, but it may never reach the recipient’s device.
Example first message:
After the first compliant message, subsequent messages in the same conversation do not need to repeat the opt-in or opt-out language.
All SMS delivery is done on a best-effort basis. A successful API response means the message was accepted by AgentPhone and handed off to the downstream carrier (Sinch), but it does not guarantee delivery to the recipient’s device.
What “delivered” means: Delivery receipts indicate the message was received by the recipient’s carrier network. Once the carrier has the message, final delivery to the device depends on factors outside of AgentPhone’s control.
VoIP-to-VoIP filtering: Messages sent to other VoIP numbers (e.g. numbers provided by other VoIP providers) are frequently filtered by the receiving carrier. This is common carrier behavior and there is no error returned when it happens.
For the most reliable delivery, send messages to real mobile numbers rather than other VoIP or virtual numbers.
iMessage limits are based on whether a contact messaged you first and how recently they interacted with your number.
If you need higher inbound, follow-up, or outbound-first capacity, we recommend load balancing across multiple numbers on your end. Each additional line gives you another full set of daily limits.