# CGNAT setup recommendations for SIP trunks and VoIP devices

> Category: SIP-Trunks
> Canonical URL: https://crazytel.com.au/knowledgebase/article/cgnat-setup-recommendations

Carrier-Grade NAT (CGNAT) settings for SIP trunks and VoIP devices. When your ISP uses CGNAT, enable NAT Helper on the SIP Trunk, set a 180 second registration period, and enable Keep Alive on the VoIP device to keep registrations active and fix connectivity or registration problems.

Carrier-Grade NAT (CGNAT) allows an internet service provider (ISP) to share one public IPv4 address among multiple customers. Your modem or router may receive a private IP address while the ISP translates traffic to the shared public address.

CGNAT can cause connectivity or registration problems for SIP devices if inbound traffic does not reach the correct device.

## Recommended settings

1. Enable **NAT Helper** on the SIP Trunk. It sends server-side keep-alive packets every 30 to 60 seconds to help keep the registration active.
2. If your device allows it, set its registration period to 180 seconds (3 minutes).
3. Enable **Keep Alive** on the VoIP device to help maintain the connection between the device and the registrar.

## Devices with limited settings

Some devices, including certain FRITZ!Box models, may not allow you to change the transport type.

1. If the device does not allow you to change it, leave **Transport Type** set to *UDP*.
2. Set **Registration Length** to 60 seconds or less.

## If the issue continues

After changing the settings, test registration and place a call. If the issues continue, contact Crazytel Support.