I signed up for Vonage thinking I will install the V-Portal adapter behind my Cisco ASA 5505. I subsequently discovered Vonage requires inbound access to the V-Portal on UDP ports 10000-20000.
I have an ASA 5505 and have used it with Vonage and numerous other VoIP providers. DO NOT configure port forwarding. The ASA does SIP Inspection perfectly and will permit any required SIP/RTP flows.