VoIP Calls Dropping? How to Diagnose and Fix Disconnections

Few things are more frustrating than a VoIP call cutting out mid-conversation. You are halfway through a sales pitch or client update and suddenly — silence. The call drops, you scramble to redial, and your professional image takes a hit.

Dropped VoIP calls are one of the most common complaints businesses have about their phone systems, but the good news is that almost every cause is fixable. This guide walks you through the most likely reasons your calls are disconnecting and exactly what to do about each one.

Why Do VoIP Calls Drop?

VoIP calls travel over your internet connection as data packets. Unlike traditional phone lines that maintain a dedicated circuit, VoIP relies on a stable, consistent flow of data between your phone and the provider servers. When that flow is interrupted — even briefly — calls drop.

The root causes fall into a few broad categories:

• Network and internet issues — the most common culprit by far
• Router and firewall configuration — settings that actively interfere with voice traffic
• Hardware problems — failing phones, switches, or cables
• Provider-side issues — less common but worth ruling out

Step 1: Check Your Internet Connection

Before you touch anything else, verify your internet connection is healthy. VoIP does not need enormous bandwidth, but it is extremely sensitive to instability.

Bandwidth Requirements

Each concurrent VoIP call typically needs:

• 100 Kbps upload and download per call (using the G.711 codec)
• 30-40 Kbps per call with compressed codecs like G.729

That sounds tiny, but the issue is rarely raw speed. It is what happens to your connection under load.

The Real Metrics That Matter

Run a VoIP-specific speed test (not just a standard speed test) and pay attention to:

• Packet loss — should be below 1%. Anything above 2% will cause audible problems and dropped calls. Even 1-2% packet loss can degrade call quality noticeably.
• Jitter — the variation in packet arrival times. Keep this under 30ms; under 15ms is ideal. High jitter means packets arrive out of order, making it impossible to reconstruct audio smoothly.
• Latency — the round-trip time for packets. Under 150ms is acceptable for voice; under 80ms is preferred. Higher latency causes awkward conversational delays.

If any of these metrics are poor, you have found your problem. The fix is either improving your connection or managing how traffic flows across it.

For a deeper look at broadband options that support reliable VoIP, our UK business broadband comparison covers fibre, 5G, and leased line options side by side.

Step 2: Implement Quality of Service (QoS)

QoS is the single most impactful change you can make. It tells your router to prioritise voice traffic over everything else on your network.

Why QoS Matters

Without QoS, your router treats all traffic equally. A large file download, a Windows update, or someone streaming video will compete directly with your voice calls for bandwidth. The voice packets get delayed or dropped, and your calls suffer.

How to Set It Up

1. Log into your router admin panel — typically at 192.168.1.1 or 192.168.0.1
2. Find QoS or Traffic Management settings — the exact location varies by manufacturer
3. Prioritise SIP and RTP traffic — SIP (port 5060) handles call setup; RTP (ports 10000-20000, varies by provider) carries the actual audio
4. Set voice traffic to highest priority — above web browsing, file transfers, and streaming
5. Consider bandwidth reservation — reserve a portion of your upload bandwidth exclusively for voice

If your router does not support QoS properly, that is a strong signal you need business-grade networking equipment.

Step 3: Check Your Router and Firewall

Routers and firewalls are involved in a surprising number of dropped call issues.

SIP ALG — Turn It Off

SIP Application Layer Gateway is a feature built into many routers that attempts to help VoIP traffic pass through NAT. In practice, it almost always causes more problems than it solves:

• Calls connecting but dropping after 30-60 seconds
• One-way audio (you can hear them but they cannot hear you)
• Calls failing entirely on some handsets but working on others

The fix: Disable SIP ALG in your router settings. This resolves call-dropping issues for a significant number of businesses. Look for it under NAT, SIP, or VoIP settings in your router admin panel.

Firewall and NAT Settings

• Ensure SIP ports are open — UDP port 5060 (or 5061 for encrypted SIP) must not be blocked
• Ensure RTP port range is open — your VoIP provider will specify the range (commonly 10000-20000)
• Check NAT timeout settings — if your NAT session timeout is too short, the connection between your phone and the server drops between calls, causing registration failures and dropped calls. Set UDP timeout to at least 300 seconds.

Step 4: Inspect Your Local Network

Problems on your internal network are easy to overlook but common.

Ethernet vs WiFi

If your VoIP phones are connected via WiFi, switch them to wired Ethernet immediately. WiFi introduces variable latency and packet loss that wired connections simply do not have. Every professional VoIP deployment should use wired connections for desk phones.

Cabling and Switches

• Check Ethernet cables — damaged or low-quality cables cause intermittent packet loss. Replace any Cat5 cables with Cat5e or Cat6.
• Check network switches — unmanaged consumer switches can cause issues under load. Business-grade managed switches with PoE (Power over Ethernet) are ideal for VoIP handsets.
• Use a dedicated VLAN — separating voice traffic onto its own VLAN isolates it from data traffic and makes QoS more effective.

Step 5: Review VoIP Phone Settings

Sometimes the issue is on the handset itself.

• Check registration status — if the phone shows "unregistered" or keeps re-registering, it cannot maintain calls. This often points back to network or NAT issues.
• Update firmware — outdated phone firmware can have bugs that cause call drops. Check your manufacturer website for the latest version.
• Check codec settings — ensure the phone is using a codec your provider supports. Mismatched codecs can cause calls to connect then fail.
• Reduce registration expiry time — setting a shorter registration interval (e.g., 60 seconds instead of 3600) helps the phone maintain its connection through NAT.

Step 6: Rule Out Provider Issues

If you have checked everything on your end and calls still drop, the issue may be with your VoIP provider.

• Test from a different location — try making calls from a different internet connection (even a mobile hotspot). If calls work fine elsewhere, the issue is your network. If they still drop, it may be the provider.
• Check your provider status page — most providers publish service status updates
• Ask about their infrastructure — are they using redundant data centres? What is their uptime SLA?
• Review call logs — your provider should be able to show you server-side logs indicating whether calls are being terminated by your equipment or theirs

If your current provider cannot diagnose the issue or their infrastructure is unreliable, it may be time to look at alternatives. Our guide to hosted VoIP solutions in the UK compares the leading providers.

Common Dropped Call Patterns and What They Mean

The pattern of your dropped calls often points directly to the cause:

• Calls drop after exactly 30 seconds — almost certainly a SIP ALG or firewall issue. The initial SIP session times out because return traffic is being blocked or mangled.
• Calls drop at random intervals — likely packet loss or bandwidth contention. Run a continuous ping test to your VoIP provider server to check for intermittent drops.
• Calls drop during busy periods — bandwidth saturation. QoS will help, but you may need more bandwidth or a dedicated voice connection.
• Calls drop on specific handsets only — hardware or configuration issue on that device. Try swapping it with a working handset to confirm.
• All calls drop simultaneously — internet outage or router crash. Check your router logs for reboot events.

Preventing Future Call Drops

Once you have fixed the immediate issue, put these measures in place to keep calls stable long-term:

1. Monitor your connection continuously — use a network monitoring tool that alerts you to packet loss and jitter spikes before they affect calls
2. Keep firmware updated — on phones, routers, and switches
3. Maintain QoS settings — review them whenever you add new phones or change your network
4. Have a backup internet connection — a 4G/5G failover ensures calls continue even if your primary line goes down
5. Document your VoIP configuration — ports, codecs, VLAN settings, QoS rules. This saves hours when troubleshooting future issues

When to Call in Expert Help

If you have worked through this guide and calls are still dropping, or if you do not have the in-house expertise to configure QoS, VLANs, and firewall rules, it is worth getting professional help. A managed VoIP provider handles all of this for you — monitoring your connection, configuring your network, and resolving issues before they affect your team.