What "multi-SIM" adds over a single modem
A single GSM modem does one thing at a time on one SIM. Put several modems in one box, give each its own SIM, and you have a multi-SIM gateway: messages go out and come in on many numbers simultaneously, and your software decides which channel carries which message. The benefits are throughput (more channels, more messages per hour), resilience (a problem with one SIM does not stop the service), and flexibility (separate numbers for different branches, clients or purposes, or a mix of networks for coverage).
Routing and load balancing across SIMs
The hardware gives you independent channels; the software decides how to use them. The strategies that matter in practice:
- Round-robin: each outbound message goes to the next free channel, spreading wear and daily volume evenly across SIMs.
- Per-SIM caps: a daily or hourly limit per channel keeps every SIM inside its tariff's fair-use terms; when a SIM reaches its cap the queue moves on.
- Network matching: send to O2 numbers from O2 SIMs and to Vodafone numbers from Vodafone SIMs where on-net messages are cheaper or faster.
- Sticky routing: replies to a customer always come from the number they last heard from, so conversations stay on one thread.
- Failover: a channel that loses registration or runs out of credit is skipped automatically and retried later.
Gammu-SMSD, Kannel and commercial SMS servers all support some or all of these; the USB setup guide shows how the channels are enumerated so the software can address them.
Multi-SIM modem pool or SIM bank?
| Multi-SIM modem pool | SIM bank + remote gateways | |
|---|---|---|
| Where the SIMs live | In the modems that use them, one box | In a central bank, served over IP to gateways elsewhere |
| Typical use | SMS from one site: notifications, two-way messaging, telemetry, testing | Voice termination and SIM rotation across many sites |
| Set-up | USB to a host PC, AT-command SMS software | Network configuration, bank plus gateway units, vendor software |
| Cost | From £7.01 per channel (64-port) | Significantly higher; two classes of hardware |
| UK legal position | SIM farm under the Crime and Policing Act 2026: good reason required | Same, and SIM rotation for voice attracts particular scrutiny |
For SMS work from a single location, the modem pool is the simpler and cheaper design. SIM banks are a different product aimed at distributed voice gateways, and we do not sell them.
Sizes of multi-port GSM modem
Multi-port GSM modems are sold in a handful of standard sizes. Small units of 4 or 8 channels suit a single site sending a few hundred messages a day. 16 and 32-channel units suit busier services or organisations that need many numbers. 64-channel units are the usual top of the desktop range and are chosen by messaging-heavy operations, telemetry collectors and test labs. Our range covers 8 to 64; the comparison page sets out the numbers, and at present the 64-port model is the one available.
How a multi-SIM GSM modem is built
The 64-port unit shows the construction clearly. Each channel is a plug-in module with its own SIM holder, SMA antenna socket and power switch, so a SIM can be changed or a module replaced without disturbing the rest. A backplane connects the modules to USB hub circuitry; the chassis has two USB-B host ports (one per 32-channel bank) and USB-A ports for chaining a second unit. Mains power is 100-240 V AC at around 60 W. The host operating system sees one serial port per channel and any AT-command SMS software can drive them.
Multi-SIM gateways in the UK: networks and the law
In the UK the modules register on 2G (3G is switched off) with EE, Vodafone or O2 SIMs, not Three; and from 29 October 2026 any device using five or more SIMs is a SIM farm under the Crime and Policing Act 2026, so we verify buyers at checkout. Details: UK networks and SIMs and lawful use.
Related terms
You will also see multi-channel GSM modem, SIM bank (a different product: a remote SIM host used with separate gateways) and GSM modem pool. For the pool terminology see GSM modem pools; for the hardware/API distinction see SMS gateway hardware.