Short answer: SMS API integration connects your software to a messaging gateway over HTTP, so a text goes out the moment an event happens. With VoidFix, you POST your API key, the recipient's number, the message and a device ID to the VoidFix send.php endpoint, and the text leaves from the SIM in your Android phone.
SMS API integration is how software sends and receives text messages without anyone touching a phone. Instead of pasting numbers into a dashboard, your application talks to a messaging gateway over HTTP and triggers texts when something happens.
Done well, SMS becomes a reliable layer under your product, CRM and automations. This guide explains how requests and delivery tracking work, what the US registration rules mean for API senders, and how to connect the VoidFix Gateway API.
What does SMS API integration mean?
SMS API integration connects your own software to a messaging gateway so text messages can be sent and received programmatically. Your application makes a request to an endpoint, and the gateway handles delivery to the carrier and the handset.
An API, or application programming interface, is a defined way for two systems to talk. A REST SMS API exposes a handful of HTTP endpoints: usually one to send, one to check status and one for contacts or replies. You send a small block of structured data and get a structured response back.
The payoff is automation. When a customer orders, signs up, books an appointment or abandons a cart, your system can send the right text straight away, with no person in the loop.
What goes into an SMS API request?
Most SMS APIs follow the same shape. A typical send request needs four things:
- Authentication: an API key or token that proves your application may send. VoidFix uses a
keyparameter. - Destination: the recipient's number, ideally in international E.164 format such as +14155550123.
- Sender: the number the message comes from. With VoidFix you choose the connected device (and its SIM) with the
devicesparameter. - Body: the message text, plus optional fields for scheduling or media.
Here is a single send from the VoidFix documentation, form-encoded:
POST https://sms.voidfix.com/services/send.php
Content-Type: application/x-www-form-urlencoded
key=YOUR_API_KEY&number=+15551234567&message=Hi, your appointment is tomorrow at 10am.&devices=215Remember that sending is synchronous but delivery is not. The API accepting your request means the gateway has it. Whether the handset received it is known a moment later.
Which VoidFix API endpoints do you need?
The SMS and MMS API uses the base URL https://sms.voidfix.com and authenticates every call with your API key.
| Endpoint | Method | What it does |
|---|---|---|
/services/send.php | POST | Send a single or bulk SMS/MMS. Options include schedule, isMMS with attachments, prioritize and, for bulk, useRandomDevice |
/services/read-messages.php | GET | Get messages by ID, group ID or status |
/services/resend.php | POST | Resend messages by ID or group ID |
/services/manage-contacts.php | POST | Add a contact or unsubscribe a contact |
/services/get-devices.php | GET | List your connected devices |
/ajax/zapier/get-contacts-list.php | GET | Get contact lists |
/ajax/services/create-contacts-list.php | POST | Create a contact list |
Source: the VoidFix REST API documentation. iMessage has its own endpoints, covered in the developer hub.
How do you track delivery status and replies?
A one-way send is useful, but real integrations are conversations. You want two things back from the gateway:
- Delivery status. The phone reports whether each message was delivered or failed. You can read it with
read-messages.phpby message ID, group ID or status, and see it in the dashboard's delivery reports. - Inbound replies. Replies come back over the phone's live connection and appear in your dashboard, and in your CRM if you use a native integration such as GoHighLevel.
For webhook options beyond that, check the full SMS API reference or ask our team. Handling replies is also how you stay compliant. When someone texts STOP, unsubscribe the number (for example with manage-contacts.php) so no further marketing goes out.
Why does US A2P registration matter for API senders?
With a traditional messaging API in the US, you cannot simply start sending from a 10-digit number. You first register under A2P 10DLC: a brand, then each campaign, recorded with The Campaign Registry. Twilio's A2P 10DLC documentation says registration brings lower filtering and higher throughput, and unregistered traffic pays extra carrier fees. Its fee list includes a one-time $15 campaign vetting fee and monthly campaign fees of $1.50 to $10 (as of October 2026). For background, read our guide to A2P 10DLC registration.
VoidFix takes a different route. Messages leave from SIMs in your own Android phones, a person-to-person route, so 10DLC registration is not part of the setup. That is not an exemption from carrier rules. CTIA's Messaging Principles say automation used by a business is not typical consumer operation, and carriers can filter or suspend lines that send high volumes or unsolicited texts. Many consumer SIM plans also limit commercial bulk use. Consent, opt-outs and quiet hours still apply.
How do you integrate the VoidFix SMS API?
Connecting your app takes a few steps:
- Connect a device. Sign up, install the VoidFix Android app and link your phone using the device connection guide.
- Copy your API key. In the SMS portal, open the API section and copy the key. Use
get-devices.phpto find your device ID. - Send a test message. Make one POST to
send.phpwith a number and message. The text should arrive on a real handset within seconds. - Store the message ID. Save the ID from the response with your own record, then check its status with
read-messages.php. - Wire it into your events. Call the send endpoint where messaging matters: signups, order confirmations, reminders and follow-ups.
- Add safeguards. Handle opt-outs, retry failed messages with
resend.php, and pace large sends across your devices.
If you would rather not write code, VoidFix also connects through GoHighLevel, Zapier, Make and Zoho. The Make guide shows the same send.php call built with a no-code HTTP step.
How do you design integrations that scale?
A demo that sends one message is easy. An integration that reliably sends tens of thousands takes discipline:
- Queue your sends. Push outbound messages onto a queue rather than sending inside a web request, so a traffic spike never blocks your app.
- Avoid double texts. Keep your own reference for each send so a retry after a timeout does not text a customer twice.
- Pace and spread volume. Rotate across devices with
useRandomDevice, turn on Number Warmup for new SIMs and use Spintax so bulk messages are not identical. - Log everything by message ID. Store the gateway message ID with each event so support and analytics can trace any message end to end.
VoidFix charges a flat monthly plan with no per-message fees, so more volume does not mean a bigger bill from us. You still pay for the SIM plan in each phone. Compared with per-segment billing, businesses can save up to 80% on messaging costs.
Which use cases should you build first?
These deliver value quickly:
- Verification codes: one-time codes that gate logins and sensitive actions.
- Order and shipping updates: automatic confirmations and tracking that cut support tickets.
- Appointment reminders: a reminder the day before to help cut no-shows.
- Lead follow-up: an instant text to a new lead while interest is high.
- Bulk campaigns: promotions and announcements to a segmented, opted-in list.
Each is a small amount of work for a lasting return, and all run through the same API.
Frequently asked questions
How does the VoidFix SMS API authenticate requests?
Every request includes your account API key in the key parameter. You find the key in the API section of the SMS portal.
Can I send bulk SMS in one API call?
Yes. The send endpoint accepts a messages array, each with a number and message, and the useRandomDevice option rotates sending across your connected devices.
Can I send MMS through the API?
Yes. Set isMMS to true and pass media URLs in the attachments parameter.
Do I need to write code to connect VoidFix to my CRM?
No. GoHighLevel, Zapier, Make, HubSpot and Bitrix24 connect without code, and Zoho uses a workflow webhook. The REST API is there when you need full control.
Build it once and reuse it
SMS API integration pays off every day after you ship it. With one well-built connection, your apps send timely texts at scale without manual work, from real numbers on a flat plan. Ready to wire texting into your software? Start your free 7-day trial, browse the developer hub, or contact our team to talk through your integration.
