@Liftboy I found this comment on another froum If the FritzBox units that Zen ship are working in the same way as an ISP a bunch of my German users are using is, then it's probably not going to work....

Fozzie Bear wrote 1) If I purchase a ER7212PC can I set this up behind the existing router in order to configure it before the ISP put my existing NTE into bridge mode? I appreciate it will be double...

@Liftboy A Grandstream WLan phone has the same problems as the Fritzbox. The Grandstream does have the ability to send a SIP debug log to a syslog server I believe. As said earlier, I suspect its...

@Liftboy I'm assuming the SIP registration is failing ? does the Fritzbox have any logging that would be useful ?

@Liftboy As I understand , the Fritzbox is also a router so you may well have a double NAT scenario. Can you explain how the Fritzbox is configured ?

@Liftboy Liftboy wrote I allready enabled SIP ALG. In my experience , most SIP ALGs are flawed and are best disabled. Most SIP endpoints can handle a connection behind NAT without the use of an alg...