In short: Hosted VoIP is business calling where the phone system runs in a provider's data centre and calls travel as data over your internet connection. Good call quality depends on enough upload bandwidth, a network that prioritises voice (QoS), low jitter, latency and packet loss, correct firewall settings, and a plan for power cuts.
How VoIP carries a call
When someone speaks into a VoIP phone or app, a codec turns the sound into small packets of data, typically one packet every 20 milliseconds in each direction. Two protocols do the work:
- SIP (Session Initiation Protocol) sets up, transfers and ends calls. It is the signalling: who is calling whom, and on which device.
- RTP (Real-time Transport Protocol) carries the audio packets themselves once the call is connected.
With hosted VoIP, your phones and apps register to a phone system in a data centre. That system routes internal calls between users and connects external calls to the public phone network through the provider's SIP trunks. The same technology underpins every platform on this site, from Forever UC to hosted 3CX. For the service side (features, platforms and what we manage), see our hosted phone system page.
Bandwidth and upload capacity
Each call uses bandwidth in both directions at once, so upload speed matters as much as download. Many broadband services are asymmetric, with far less upload than download, and that upload is shared with cloud backups, video meetings and file sync.
How much a call needs depends on the codec. Cisco publishes per-call figures: G.711 and G.722 use about 87.2 kbps per call including Ethernet overhead, and G.729 about 31.2 kbps, at the usual 20 ms packetisation. Apps often use Opus, which adapts its bit rate to the connection. We show the full working, and how to size by simultaneous calls, in how many SIP channels do you need?
Plan for your busiest moment, not an average day, and leave headroom for everything else that uses the connection at the same time.
QoS, VLANs and your office network
Voice is sensitive to delay in a way email and web pages are not, so the network should give it priority:
- QoS (quality of service) marks voice packets so switches, Wi-Fi and the router send them first. Voice is normally marked DSCP EF (46).
- A voice VLAN keeps phones on their own network segment, which makes QoS easier and separates phones from general PC traffic. Desk phones can learn their VLAN automatically through LLDP-MED or DHCP options.
- PoE (Power over Ethernet) switches power desk phones through the network cable, so there are no power bricks on desks and one UPS can keep phones running.
- Wi-Fi needs good coverage and roaming for softphones and mobile apps used around a building. A site survey helps in larger premises.
QoS on your own network cannot control the public internet, but it stops a large download in the office from crushing calls on your own line.
Call quality: jitter, latency and packet loss
Three measurements decide whether a VoIP call sounds clear:
| Factor | What it is | What you hear when it is bad |
|---|---|---|
| Latency | The time a packet takes to travel from speaker to listener | Talking over each other and awkward pauses |
| Jitter | Variation in packet arrival times | Choppy or robotic audio |
| Packet loss | Packets that never arrive | Clipped words and gaps |
The ITU-T G.114 recommendation, as Cisco summarises it, treats one-way delay of up to 150 ms as acceptable for most user applications. Phones use a small jitter buffer to smooth out variation, but it cannot fix a congested connection. Before you switch we test your connection, and our features glossary defines these and related terms such as codec and QoS.
Broadband or leased line?
Both can carry business VoIP well. The question is how much else shares the connection and how much resilience you need.
| Connection | Strengths | Watch out for |
|---|---|---|
| Business broadband (full fibre, FTTP) | Widely available, good upload on full fibre, cost-effective for smaller sites | Contended capacity; upload is lower on part-fibre services; repair times vary |
| Leased line | Dedicated, uncontended, the same speed up and down, with a service level agreement | Longer lead time to install; usually a bigger commitment |
| 4G or 5G backup | Automatic failover if the main line fails | Variable capacity; check coverage at the site |
Busy offices, contact centres and multi-site organisations often choose a leased line with broadband or 4G as backup. Our sister company also covers BT leased lines.
Firewalls, SIP ALG and power cuts
Two practical issues cause most early VoIP problems:
- SIP ALG. Many routers have a SIP application layer gateway that rewrites SIP messages. It often causes one-way audio, dropped calls or phones that will not register. 3CX, for example, tells customers to make sure SIP ALG and any SIP helper are disabled, and we check the guidance for whichever platform you choose. Firewalls also need the right ports and session timers for your platform.
- Power. As Openreach points out, digital phone lines need power to work, so VoIP phones and routers stop in a power cut unless they have backup. Anyone who depends on a landline should tell their provider, who can offer battery backup. Many organisations add a UPS for the router, PoE switch and key handsets. Mobile apps keep working on mobile data during a local power cut.
Alarms, lifts, door entry and card terminals that still use a phone line need their own plan. Our PSTN switch-off checklist covers them. When you are ready, contact us and we will check your connection and network first.
Why choose your cloud phone system with Forever Systems
We are vendor-neutral. Forever Group supplies and supports several cloud phone platforms, so we recommend the one that fits how your organisation works rather than the one we happen to sell.
- Side-by-side comparison of hosted 3CX, Microsoft Teams Phone, Forever UC, Forever Teams and Webex Calling for your users
- Contact centre and call recording options matched to your call volumes and CRM
- Number porting, handsets and set-up handled for you, including the move off analogue and ISDN lines
- Connectivity checked so call quality is right from day one
- UK support for moves, adds, changes and faults
- For BT Cloud Voice, see our BT Business Partner site cloudvoice.cloud
We work with businesses, schools, charities and public sector organisations. We do not sell consumer phone services.
Want a vendor-neutral cloud phone system comparison and quote?
Tell Forever Systems what you need and we will come back to you by email with options, not a hard sell.
Contact usFrequently asked questions
Does VoIP work on any broadband?
It works on most business broadband, but quality depends on upload speed, how busy the connection is and how the office network is set up. Full fibre or a leased line gives the most headroom. We test your connection before you switch.
How much bandwidth does a VoIP call use?
It depends on the codec. Cisco's published figures are about 87.2 kbps per call for G.711 and about 31.2 kbps for G.729, including Ethernet overhead, in each direction. Multiply by your busiest number of simultaneous calls and add headroom for other traffic.
Should I turn off SIP ALG on my router?
In most cases, yes. SIP ALG often interferes with VoIP signalling and causes one-way audio or registration failures. Check your platform's guidance first, as firewall settings vary by provider.
Will VoIP phones work in a power cut?
Not without power. Desk phones, PoE switches and routers need mains power or a UPS. Mobile apps keep working on mobile data. If anyone relies on a landline, for example to make emergency calls, tell your provider, who can offer battery backup.
What is the difference between hosted VoIP and SIP trunks?
With hosted VoIP the provider runs the phone system for you. With SIP trunks you keep your own phone system and the trunks only replace the lines that connect it to the phone network.