This guide covers Fritz!Box routers and SIP-based VoIP.

Fritz!Box is designed for consumer VoIP and works well with its own telephony features, but third-party PBX or hosted VoIP can sometimes clash with built-in behaviour.

Before you start

• Access to Fritz!Box UI
• SIP provider details

Ports used by VoIP

• SIP: 5060
• RTP: 10000–20000

Configuration

1. Port sharing (if required)

Go to:

Internet → Permit Access → Port Sharing

Add rules for:

• SIP
• RTP

(Only needed in some setups, especially with on-site PBX)

2. Disable SIP interference (if options available)

Depending on model/firmware:

• Disable SIP ALG or similar
• Avoid “helper” behaviour

3. Prioritisation

Go to:

Internet → Filters → Prioritisation

Add:

• VoIP devices or PBX

Set to:

• Real-time priority

4. Built-in telephony considerations

If using Fritz!Box telephony:

• Avoid conflicting SIP registrations
• Don’t mix internal and external SIP endpoints unnecessarily

Good practice

• Keep things simple — Fritz!Box doesn’t like complex routing
• Avoid double NAT
• Be cautious mixing PBX + Fritz telephony

Troubleshooting

One-way audio

Check:

• RTP handling
• NAT issues

Registration conflicts

Often happens if Fritz telephony also tries to use the same SIP account.

Calls not working properly

Check:

• Port sharing
• Prioritisation

Quality issues

Check:

• DSL/connection quality
• Traffic prioritisation