Bulk SMS API: How to Send High-Volume Text Messages in the US

Quick answer: what a bulk SMS API actually gives you

A bulk SMS API sends large volumes of text messages programmatically, usually by accepting a list of recipients in one request or a rapid series of requests, then queueing and pacing delivery. The part that decides whether a bulk send succeeds is not the API — it is throughput: how many messages per second the carriers will accept from your number type and your registered trust tier. Getting 100,000 messages delivered in an hour is a question of number type, 10DLC trust score and queue design, not of how fast your code can loop.

The three number types, and what they mean for volume

• 10DLC long codes. Throughput is set by your brand's trust score, from a trickle at the unvetted default up to hundreds of message parts per minute at the top tiers. You can add numbers to a campaign to raise aggregate throughput, within limits. See how trust scores set throughput.
• Toll-free. Higher throughput than most 10DLC tiers out of the box and a simpler verification process, which makes it a common choice for bulk transactional sending.
• Short codes. The highest throughput available and the best filtering tolerance, at the highest cost and with the longest lead time to provision. Justified when volume is genuinely large and sustained.

Our comparison of short code vs 10DLC vs toll-free works through which to choose for a given volume and use case.

Sizing numbers to volume

The arithmetic is simple and people skip it. Take your target: 100,000 message parts to go out inside a two-hour window is about 14 parts per second sustained. Divide that by the per-number throughput your tier allows, and that is how many numbers the campaign needs — or the signal that you need toll-free or a short code instead. Do this before the campaign, not during it.

Two cautions. Spreading one programme across many long codes to manufacture throughput is snowshoeing, and carriers filter for it; numbers must be legitimately attached to the registered campaign. And every number you attach still costs monthly rental, which is part of the real cost of a bulk programme.

What actually gets bulk sends filtered

• Content drift. Traffic that stops matching the sample messages you registered. This is the most common cause of a programme that worked for months quietly degrading.
• Link shorteners. Public shorteners are heavily filtered. Use links on your own registered domain.
• No opt-out on the first message of a campaign burst, or STOP handling that is not automatic.
• Stale lists. Sending to numbers that have churned, disconnected or never really consented drives complaint and unknown-subscriber rates, which is exactly what carrier filters watch.
• Bursting from cold. Going from zero to full volume on a new number looks like spam. Ramp.

Designing the send

1. Queue, do not blast. Submit at or just under your permitted rate and let the queue drain. Hammering past your limit produces rate-limit errors, not faster delivery.
2. Handle errors per message. Read the error codes and act on them: retry transient failures, suppress hard failures permanently.
3. Consume delivery receipts. A 202 from the API means accepted, not delivered. Track the DLR to know what actually landed.
4. Suppress on STOP immediately, across every campaign, not just the one that triggered it.
5. Ramp new numbers over several days rather than opening at full volume.
6. Send in the recipient's timezone and inside legal hours. This is a compliance requirement, not a nicety.

Bulk SMS API versus a mass-texting tool

If your sends are driven by an application — an order state changes, an appointment moves, a balance is due — use the API. If they are driven by a person choosing a list and writing a message, a mass-texting interface is faster and needs no engineering. Most businesses end up wanting both, on the same numbers and the same registration. Our mass texting product covers the human side; the API covers the programmatic side.

Before your first bulk send

Registration has to be complete and honest, your throughput tier has to match your target window, your list has to be genuinely opted in, and your content has to match your registered samples. Get those four right and bulk sending is unremarkable. Get any one wrong and no amount of retry logic will rescue the campaign. Start with the A2P 10DLC requirements if registration is not done.

Tell us your volume and your delivery window and we will size the number type, throughput tier and ramp for you before you send anything.

People also ask

What is a bulk SMS API?

A bulk SMS API is an interface for sending large volumes of text messages programmatically, accepting many recipients per request or a rapid stream of requests and then queueing and pacing delivery to stay within carrier limits. The practical constraint is throughput — the messages per second your number type and registered trust tier permit — rather than the API itself.

How many SMS can I send per second?

It depends on number type and registration. Unvetted 10DLC brands get conservative default throughput; vetted brands get progressively more, and adding legitimately registered numbers raises aggregate throughput. Toll-free generally allows more out of the box, and short codes the most of all. Calculate the messages per second your delivery window needs, then choose the number type that supports it.

Why are my bulk SMS messages not being delivered?

The usual causes are content that no longer matches your registered sample messages, public link shorteners, missing or non-automatic STOP handling, sending to a stale list with high unknown-subscriber rates, spreading one programme across many long codes (snowshoeing), or bursting a new number to full volume from cold. Check delivery receipts and error codes rather than assuming the messages went out.

Is bulk SMS legal in the US?

Yes, with consent. TCPA requires prior express written consent for marketing texts, opt-out requests must be honoured promptly, and sending is restricted to reasonable local hours in the recipient's timezone. Separately, carriers require A2P 10DLC, toll-free or short code registration before they will accept the traffic at volume.