Zyxel 600 SIP problems

If you are using Zyxel 600 series Routers and you have a VOIP PBX you are surely experiencing errors on registrating VOIP accounts, ie Eutelia, voipstunt, voipcheap, etc.

In your VOIP box you can see that there is no answer to the registration requests.

In asterisk, for example, in the CLI you can see:

sip show registry

asterisk*CLI> sip show registry
Host Username Refresh State Reg.Time
voip.eutelia.it:5060 080xxxxxx 120 Request sent

The solution is pretty simple, you need to disable SIP ALG in nat settings section of the router 😉