Definitions
GSM modem pool: a piece of hardware. Many GSM modems in one chassis, each with a SIM, presented to a computer as serial ports. It does nothing until software drives it.
SMS gateway: a function. Anything that takes messages from your systems and gets them onto the mobile network (and back). It can be a cloud service with an API, a small appliance with one SIM and a web interface, or a modem pool plus software.
So a modem pool is an SMS gateway once it is running, which is why the 64-port unit on this site is legitimately described as a 64-port SMS gateway. But not every SMS gateway is a modem pool.
Side by side
| GSM modem pool (hardware gateway) | Cloud SMS gateway (API) | |
|---|---|---|
| What you buy | A device, once | A subscription or per-message credit |
| Numbers | Your own SIMs, your own numbers | Shared short codes, alphanumeric IDs or rented virtual numbers |
| Per-message cost | SIM tariff only; often inclusive | Typically 2p to 5p per UK message |
| Inbound replies | Arrive on your channels, your software | Delivered by webhook, if the number supports it |
| Internet needed | No | Yes |
| International reach | Limited to the SIMs you hold | Global routes |
| Setup | Hardware, SIMs, SMS software | API key and code |
| Legal position (UK) | Multi-SIM device: good reason required from 29 Oct 2026 | Provider's terms; sender ID rules |
Which to choose
Choose the hardware gateway when volume is steady and mostly UK, when replies matter, when you want your own numbers, or when the messages are operational (alarms, telemetry, dispatch) and must not depend on an internet connection or a third party. Choose an API when volume is spiky or small, when you send to many countries, or when you cannot host equipment.
Many organisations run both, using the pool for the bulk of domestic traffic and the API for overflow and overseas. Whichever you pick, sending must be lawful and consented; the lawful use page explains what we require of buyers.
Related terms
GSM gateway usually means a voice device bridging GSM and VoIP; it is a different product. SIM bank hosts SIMs remotely for gateways elsewhere; also different. Multi-SIM SMS gateway and bulk SMS modem are marketing terms for a modem pool. Our GSM modem pool page and SMS gateway hardware page cover the two sides in more depth.