Hi 

I am having trouble getting my VOIP back up and running after installing a CISCO ISR.

I got sick of my fritz box telling me what I was and was not allowed to do with my network so I got hold of a CISCO 800 Series integrated router. I have my fritz box in "internet connection sharing" mode, (which I assume is more or less a fancy way of saying bridge.) with the lan plugged into a lan port on my 800 Series, I am using the fritzbox for wireless and also as an ATA gateway for my phone.

I have internet all running no worries but cant get VOIP to work properly.

I can ring out but after a minute or two the phone will just go blank, I have been told it hangs up on the other end but just goes completely silent on my end, no one is able to ring in it just goes to answer phone after 15 secs silence.

I am almost 100% sure this is a porting issue as the 800 Series is NATing although I am a little confused as to why I can ring out but only for a minute or two. tho I am assuming this is something to do with re-registration during a call  ?

Currently I have my fritz statically assigned a ip address on my LAN Subnet and have port 5060 TCP and UPD forwarded to it but it is still not working I know that there are some RTSP ports involved but I am quite new to VOIP and am not really sure what they are, or if that is event the problem.

I would really appreciate any help from some VOIP Guru's.

Please refrain from telling me to put my fritz-box back on edge of network or telling me to get something else. this must be possible as is as the it more or less says in the fritzbox that "Internet Connection Sharing" is for this purpose.

Many thanks in advanced

PS. Im not a complete network noob so don't worrie to much about talking technical.