GSM gateways
GSM GatewayAlso known as VoIP GSM gateway, Mobile SIP gateway, Cellular gateway
Definition
A GSM gateway is a device that connects a VoIP or telephone system to a mobile network using one or more SIM cards. It converts calls between SIP/RTP on the IP side and the mobile radio network on the other, so a PBX can place and receive calls on mobile lines as if they were ordinary trunks.
How a GSM gateway works
On the IP side the gateway behaves like any other SIP endpoint or SIP trunk: your PBX or softswitch sends an INVITE, the gateway authenticates it and picks a free mobile line. On the mobile side it dials the number through the SIM’s home network, just as a handset would. Once the call is answered, audio flows as RTP packets between the PBX and the gateway, and as mobile radio voice between the gateway and the network.
Inbound works in reverse: a call to the SIM’s mobile number rings the gateway, which forwards it to a configured extension, IVR or ring group over SIP.
- Signalling: SIP INVITE, 180/183 progress, 200 OK and BYE map to mobile call set-up, ringing, answer and release.
- Media: the voice codec on the trunk (typically G.711 or G.729) is transcoded to and from the mobile codec.
- Status: signal strength, registration state and balance are reported so the gateway can skip unhealthy lines.
Common, lawful uses
Businesses use GSM gateways to give a desk phone system access to mobile tariffs and mobile numbers, to provide a backup route when a fixed line fails, to reach sites where no fixed line is available, and to present a local mobile number to customers. Every deployment must follow the SIM operator’s terms and local telecom regulation; using gateways to bypass international interconnect (see SIM box) is illegal in most countries.
How it relates to GSMCalls
Traditional GSM gateways are dedicated boxes with built-in radio modules. GSMCalls instead turns ordinary mobile phones into managed SIP gateways: phones connect over Bluetooth (Wireless Connect), through the Smart Bridge microcontroller with HD G.722 audio and no PC, or over USB. Trunks, routing, CDRs and prepaid billing for every site are managed from one cloud platform.
Frequently asked questions
What is the difference between a GSM gateway and a SIP trunk?
A SIP trunk carries calls over the internet to a VoIP carrier. A GSM gateway carries calls from your phone system out over mobile SIM lines. Many businesses use both, with routing rules deciding which calls take which path.
Does a GSM gateway work with 4G-only networks?
Classic 2G/3G gateway modules stop working when those networks are switched off. Gateways that use modern phones can place calls over VoLTE or Wi-Fi Calling, which keeps them usable on 4G and 5G networks.
Is a GSM gateway legal?
Yes, for authorised uses such as business mobile access, backup lines and local presence, within the SIM operator’s terms. Using gateways to terminate international traffic as local calls (SIM-box bypass) is illegal in most countries.
Can a GSM gateway connect to FreePBX, Asterisk or 3CX?
Yes. A GSM gateway presents itself as a standard SIP trunk or peer, so any SIP PBX including FreePBX, Asterisk and 3CX can route calls to it.