Notes on mobile SIP gateways
Practical guides from the people who build and run them — 19 articles on GSM gateways, SIP, mobile networks and compliance.
What Is a GSM Gateway? The Complete Guide (2026)
A GSM gateway connects a SIP phone system to mobile networks. How hardware, phone-based and cloud gateways work, codecs, legal use and how to choose.
Read the article →How a Mobile SIP Gateway Works
From the INVITE on your softswitch to audio on a mobile network — every hop, and where quality is won or lost.
GSM Gateway vs Traditional VoIP Gateway
Where phone-based gateways fit next to hardware chassis — and where they don’t.
USB vs Bluetooth Mobile Gateway Deployments
Cabling, density, replacement and audio: a practical comparison.
How to Build a Multi-Site SIP Gateway Infrastructure
Routing, monitoring and failover when your phones live in six cities.
Understanding GSM Gateway CDRs
Billable time, cause codes and the numbers that explain ASR and ACD.
Managing Mobile Gateway Minutes and Usage
Profiles, allocations and alerts that stop surprises at month end.
What Is a Mobile-to-SIP Gateway?
A plain-language introduction for teams new to mobile voice.
Can an Android Phone Be a GSM Gateway? (2026 Guide)
Yes, with caveats. How to get call audio in and out of an Android phone — USB, Bluetooth HFP or a USB-C bridge — plus rooting, PCs and reliability.
Wholesale VoIP & Call Termination Explained
How carriers buy and sell voice: origination vs termination, A–Z routes, ASR/ACD/PDD, Class 4 switching — and why GSM or Android termination is illegal.
What Is a SIM Box? How SIM-Box Fraud Works and Why It’s Illegal
A SIM box turns internet calls into local mobile calls. How SIM-box fraud works, who it harms, how operators detect it, and the lawful alternatives.
How to Connect a GSM Gateway to FreePBX (Step by Step)
Add a GSM gateway to FreePBX 16 or 17 as a PJSIP trunk: credentials, transport, codecs, outbound and inbound routes, plus fixes for 401s and one-way audio.
VoLTE, Wi-Fi Calling and the 3G Shutdown: What It Means for GSM Gateways
2G and 3G are being switched off. What VoLTE, IMS and Wi-Fi calling mean for GSM gateways, why 3G dongles fail, and why phones handle HD voice well.
SIP Trunks, DIDs and DOD Explained
What a SIP trunk is, how DID numbers route inbound calls, how DOD handles outbound, and how porting, caller ID and gateways fit together.
G.711 A-law vs µ-law (and When to Use G.729, G.722 or Opus)
How G.711 A-law and µ-law differ, where each is used, and when G.729, G.722 HD or Opus is the better VoIP codec for trunks and GSM gateways.
Class 4 vs Class 5 Softswitch: What’s the Difference?
Class 4 softswitches route wholesale traffic between carriers; Class 5 softswitches serve end users. How they differ in routing, billing and features.
Telecom Billing for VoIP and GSM Gateways: CDRs, Rating and Prepaid
How VoIP telecom billing works: CDR fields, rating and rounding, prepaid vs postpaid, ASR/ACD reporting, fraud controls and what to look for in software.
SIM Bank vs Remote SIM Gateway: How Centralised SIM Management Works
What a SIM bank and a remote SIM gateway are, how they connect over IP, legitimate uses, carrier and legal limits, and how phone-based platforms compare.
Securing SIP: TLS, SRTP, ACLs and Fraud Prevention for Gateways
A practical SIP security guide: TLS on port 5061, SRTP with SDES or DTLS, ACLs, strong credentials, rate limits, fail2ban and CDR monitoring for gateways.