I have vonage as well. Originally they sent me a Linksys RPT103
combination router/firewall/sip. Either I had a defective model, or it
was just a piece of crap. Anyway, now I have a Motorola VT103 which
works great. I put this infront of the firewall. Phone works great,
interenet works great. Basically my LAN looks like this

cable---cablemodem---VT103----m0n0wall----switch

btw..Ugo, if you only hearing part of the conversation, there are
ports you need to open up inbound. The Vonage website KB and/or forum
explains this. Vonage has something like the microsoft update agent
that will need access inbound to the sip.

~don