IP telephony: the 10 mistakes that ruin call quality, and how to avoid them

“Since we moved to VoIP, calls keep dropping.” When we take over a struggling IP telephony installation, the PBX is rarely the culprit. The network carrying it almost always is. Here are the ten mistakes we find, from most to least common, with the fix.

1. SIP ALG enabled on the router

This feature, meant to help voice cross NAT, rewrites SIP packets and breaks registrations: one-way audio, ringing that continues after pickup, phones that unregister. Fix: disable SIP ALG on the carrier router and on the firewall. It is the first setting we check.

2. No voice prioritisation

Without QoS, a Windows update or a cloud backup crushes voice packets. Fix: mark voice traffic DSCP 46 and prioritise it on the firewall, switches and WiFi, with about 100 kbit/s reserved per simultaneous call.

3. Phones and computers on the same network

Broadcasts, ARP storms, or simply an infected laptop saturating the network, and voice suffers. Fix: a dedicated voice VLAN with its own rules, which also simplifies security, as described in the 10 FortiGate settings after install.

4. The PBX behind a consumer box

Dynamic IP, sloppy NAT, no repair guarantee. Fix: a business link with a fixed IP, or a PBX hosted by us, and in every case a firewall you control, see business fiber in Morocco.

5. A single internet link

Fiber cut, telephony cut, and customers reach voicemail. Fix: 4G or 5G failover with automatic switching, and voice routed over the most stable link with SD-WAN.

6. WiFi phones on office WiFi

WiFi shared with laptops and smartphones causes jitter and packet loss. Fix: prefer wired for desk phones, and for mobiles a prioritised voice SSID on properly sized access points, see high-density enterprise WiFi.

7. Insufficient PoE budget

Twenty phones on a PoE switch rated for twelve: the last ones plugged in reboot randomly. Fix: compute total power, keep 30% margin, and choose PoE+ for phones with colour screens or expansion modules.

8. The wrong codecs

G.729 saves bandwidth at the cost of quality, G.711 does the opposite, Opus adapts. Fix: G.711 or Opus internally and over fiber, G.729 reserved for constrained 4G links.

9. No monitoring

You learn about the problem from customers who can no longer reach you. Fix: monitor the PBX, SIP trunks and call quality, measuring jitter, latency and loss, with alerts above 30 ms jitter or 1% loss.

10. A poorly chosen SIP carrier

A cheap SIP trunk with poor international routes, or no reachable technical support. Fix: a carrier certified for your PBX, 3CX or Yeastar, with portable Moroccan numbers.

The half-day diagnosis

A telephony audit measures these ten points, with traffic captures during problem calls. In most cases three fixes are enough: SIP ALG, QoS and voice VLAN. That is the starting point of our IP telephony offer, and choosing the PBX itself is covered in 3CX, Yeastar or Teams Phone.