Sorry, omdat ik in het engels doorgaan moet, maar mijn nederlands is helaas nog steds niet goed genoeg voor zulke dingen....
I am trying to somehow get both, a phone connected to my Telfort ADSL modem (Zyxel with WiFi), and another SIP phone (Grandstream) working in parallel - so far without success :-(
As far as my knowledge of the SIP standard goes (IETF RFC3261), it is possible for multiple SIP endpoints to register at the same time.
It should therefore in principle be possible to activate both, the SIP stack inside the Zyxel modem which serves the telephone ports, AND a seperate SIP telephone to initiate/receive telephone calls.
The SIP registrar/proxy server would then "fork" any incoming calls to both receiving contact addresses, sequentially or in parallel, causing either both telephones to ring sequentially or one after the other.
As I said, so far I have not been able to register the second device, ie the SIP phone. Strangely, I have not been able to even activate this device as the only device, i.e. with the Zyxel phone service de-activated and all respective ports being forwarded. Although this might indicate that there is a problem with the telephone itself, this is definetely not the case, since the phone supports multiple SIP providers at the same time, and is successfully registered/using another SIP provider. Still, I would also like to provision a Telfort account in the SIP phone, so I can make local calls using my Telfort phone subscription.
Has someone made similar experiences or experimented successfully with a seperate or an additional SIP phone? Any ideas why I cannot register with the Grandstream despite using the exact same settings also used by the Zyxel in successfully registering to the network are very much appreciated.
I also thinking the problem lies in portmapping. Fact is that your sip ports on the modem are not present behind the nat filter and firewall but before it, if all is ok.
I don't know which Grandstream you have, they are great phones, but most models that I know personally are connected to the network by UTP (RJ45) and not by telephone jack (RJ11) on a voip port.
As far as I know this also means you can not connect your Grandstream to the voip port of a Zyxel modem.
That also means that your Grandstream is operating on the network behind a nat firewall and portmapping will be necessary.
I've got a Greandstream GXP-2000 working with several other phones, but reason for that is that I'm running my own Asterisk voip server.
This type Grandstream has port options which can be set to be used.