VoIP explained: how internet calling works and what it needs

VoIP vs SIP trunking

Hosted VoIP replaces your phone system entirely — the exchange lives in the cloud and your handsets connect to it.

SIP trunking keeps your existing on-site phone system but replaces the old ISDN lines feeding it with internet-delivered channels. It's often the cheaper route if your PBX is modern and you want to extend its life.

What VoIP needs from your network

• Roughly 100kbps per concurrent call in each direction, so 10 simultaneous calls needs about 1Mbps up and down of headroom
• Low jitter and packet loss — quality problems are almost always jitter, not bandwidth
• QoS on the router so voice traffic is prioritised over file downloads
• Wired handsets where possible; Wi-Fi calling needs a well-designed wireless network
• A router that handles SIP correctly (badly implemented SIP ALG causes one-way audio)

Troubleshooting call quality

• Choppy audio: usually jitter or contention — check QoS and whether backups run during office hours
• One-way audio: usually NAT or SIP ALG on the firewall
• Echo: normally a headset or handset volume issue, not the line
• Dropped calls: check for a router rebooting or an unstable broadband line

The cost picture

VoIP typically removes line rental entirely and reduces per-minute costs, with many plans bundling UK landline and mobile minutes. The saving over ISDN is usually significant, but the real benefit is flexibility: adding a user is a licence change, not an engineer visit.