Overview

This guide covers how to configure DrayTek routers for SIP-based VoIP.

DrayTek devices are very commonly used in SMB environments, but they can be a bit sensitive when it comes to SIP — especially around NAT handling and ALG features. A default setup will often “sort of work” but give you intermittent issues like one-way audio or dropped calls.

Before you start

You’ll need:

• Access to the DrayTek web interface
• SIP provider details (especially IP ranges if available)
• Knowledge of whether the router is directly on the internet or behind something else

If there’s another router or ISP modem in front, double NAT is something to keep in mind.

Ports used by VoIP

• SIP: 5060 (UDP/TCP), sometimes 5061 (TLS)
• RTP: typically UDP 10000–20000

These need to pass cleanly in both directions.

Configuration

1. Disable SIP ALG

Go to:

NAT → ALG

Disable:

• SIP ALG

Why this matters:
DrayTek’s SIP ALG is known to cause problems. It attempts to rewrite SIP headers but often breaks how the PBX or phones handle NAT, leading to audio or registration issues.

2. Check NAT behaviour

DrayTek routers can use different NAT types depending on configuration.

For VoIP:

• Avoid symmetric NAT
• Ensure the router is using standard outbound NAT
• If available, use “Full Cone” or similar behaviour

You don’t normally need port forwarding for hosted VoIP.

If you’re running an on-site PBX:

• Consider static port mapping
• Make sure the PBX knows its public IP (or uses STUN)

3. Firewall rules

DrayTek firewalls are usually permissive outbound, but confirm this.

Go to:

Firewall → Filter Setup

Make sure:

• UDP 5060 is allowed outbound
• UDP 10000–20000 is allowed outbound

If rules are in place, ensure they’re not blocking VoIP traffic.

4. QoS (Bandwidth Management)

This is worth enabling if the connection is shared with other users.

Go to:

Bandwidth Management → QoS Setup

• Enable QoS
• Add a class or rule for:
  • UDP traffic
  • RTP port range

Set to:

• High priority (Class 1)

5. SIP Helper (if present)

Some models include SIP helper features separate from ALG.

If present:

• Disable SIP helper / packet inspection features

Good practice

• Use a static IP if possible
• Keep firmware updated (DrayTek often fixes VoIP behaviour in updates)
• Don’t rely on “auto VoIP optimisation” — configure it properly instead

Troubleshooting

One-way audio

Very common on DrayTek when NAT isn’t behaving.

Check:

• SIP ALG is definitely disabled
• NAT type isn’t symmetric
• RTP ports are allowed

Calls dropping after 20–30 seconds

Almost always SIP ALG or NAT timeout.

Check:

• ALG is off
• Keepalives / session timers (on PBX)

Phones not registering reliably

Look at:

• WAN connection stability
• DNS resolution
• Firewall rules

Poor quality

Usually bandwidth-related.

Check:

• QoS is enabled
• No large uploads saturating the link