As we do NOT want to provide a central RTP proxy, we are looking for a simple SIP proxy (in hardware) to install in small/medium offices to simplify/bypass all NAT problems.

It should:

1) Work as a SIP proxy server: should use a STUN server to bypass all NAT problems (obviously a GOOD implementation of STUN) and forward all SIP requests to a downstream proxy.

2) Work as a registrat proxy: short-circuit all INVITEs between internal UACs (to bypass all hairpin problme of the NAT) and forward all SIP requests to a downstream proxy.

...and optionally...

3) Handle all the authentication requests from the downstream proxy.

In this way the configuration of the phones is very simple, only phone number and sip proxy, no STUN server, no hairpin problems, etc...

If 3) is implemented, they don't even have to know the username/password used to authenticate (so they cannot make free phone calls from home!).

Does something like that exist? I mean, something that costs around a couple hundred of euro???

Thanks.