Telecoms glossary • R
Router
Every voice packet in the office passes through it. A large share of faults blamed on the provider are settings on this box.
What is a router?in VoIP and business telephony
1. A router is the device that moves traffic between your office network and the internet, deciding where each packet goes and applying the rules you set on the way. On a voice site it also translates internal addresses to a public one, enforces call priority and controls which ports and protocols are allowed through.
2. It is the single place where the three classic voice faults are created and cured, so a fault that appears across every handset at once is worth investigating here before anyone phones the provider.
What the router does to a call
A handset does very little. The router does almost everything that decides whether a call registers, connects and carries sound.
- Address translation
- NAT shares one public address across every device. It also rewrites the addresses inside the conversation, which is where one-way audio comes from when the media path and the signalling path disagree.
- Session tracking
- Registrations are held open by a table entry with a timeout. Set the timeout shorter than the handset’s refresh interval and phones drop off the platform at regular, oddly precise intervals.
- Queueing
- The uplink is where congestion happens, so the uplink is where the QoS priority queue belongs. A router that cannot queue on DSCP cannot protect a call.
- Filtering
- The built-in firewall decides whether SIP on 5060 and 5061 and the media range get through. 3CX, for example, expects UDP 9000 to 10999 for audio; other platforms commonly use 10000 to 20000.
- Capacity
- Concurrent sessions, not throughput, is the limit that bites. A unit sized for browsing can hold thousands of short-lived web sessions and still struggle with long-lived voice sessions plus cloud applications at month end.
Turn SIP ALG off before anything else
SIP ALG, sold as a helper, is the most reliable cause of unexplained voice faults on a small office router. It inspects SIP messages and rewrites the addresses inside them, and vendor implementations do it badly enough that the guidance from platform vendors is simply to switch it off.
3CX’s firewall documentation says it plainly: use a router or firewall without a SIP helper or SIP ALG, or a device on which SIP ALG can be disabled. OnSIP lists the symptoms it produces: incoming calls that fail, phones that unregister themselves, dead air and choppy sound. Those are the same symptoms an office will normally report as a provider problem.
The setting hides under different names. Look for SIP ALG, SIP helper, SIP transformations or VoIP passthrough in the firewall or NAT section. Switch it off, reboot the handsets so they re-register cleanly, and retest before changing anything else.
What to look for in a router for a voice site
Specify capability rather than brand. Any unit that does these five things properly will carry a South African office of thirty phones.
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DSCP-based queueing on the WAN interface
Not a slider marked “prioritise voice”. A policy that matches EF, decimal 46, and puts it in a policed priority queue on the uplink.
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SIP ALG that can be switched off
If the setting does not exist in the interface, assume it is on and cannot be disabled, and choose another unit.
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802.1Q VLAN tagging
You want voice on its own VLAN so classification is a matter of which port the traffic arrived on rather than deep inspection.
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Session table headroom
Ask for the concurrent session figure, not the throughput figure. Throughput is the number vendors print; sessions is the number that runs out.
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ICASA type approval
Section 35(1) of the Electronic Communications Act means equipment used, supplied, sold, leased or hired in South Africa must be approved by the Authority. Grey imports bought online often are not, which becomes your problem rather than the seller’s.
The router supplied free with a fibre line is built for a household. It will carry a handful of phones and will usually lack DSCP queueing and a disable switch for SIP ALG, which is why a business-grade replacement is the first purchase most offices make after moving to VoIP.
Telling a router fault from a line fault
Three patterns point straight back at the box. Phones that unregister at a regular interval, minutes apart, are a session timeout or SIP ALG. Audio in one direction only, on a call that connected normally, is address translation or the media ports. Audio that degrades at busy times and recovers at lunch is queueing, or the absence of it.
A fault that is genuinely upstream looks different: it affects a single site, persists on a laptop cabled directly to the fibre ONT with the router removed from the path, and shows sustained loss over a long test window. Run that test before opening a ticket, because it is the first thing you will be asked to do anyway.
About this entry
Definitions are written for South African business buyers and checked against primary sources – provider documentation, standards bodies and ICASA – not vendor marketing. Reviews are independent and sponsors are always disclosed. Read our editorial policy and scoring methodology.
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