Hi there, I used a Vonage Line on a Norstar system and basically hooked it up to an analog trunk port the same way you'd hook up an analog POTS line. I plugged a standard RJ-11 cord into the vonage box and then stripped the other end and punched into a CO port on the Norstar.
On the negative, Vonage was ridiculously expensive for what you received. I instead switched to VOIP.MS and bought PAP2 Cisco Analog Terminal Adapters and set up 4 pots lines via VOIP.MS. VOIP.MS is so inexpensive....I now have pure SIP trunks running through a BCM50 SIP trunk ports and for $25 a month, I have 12 SIP trunk lines on my small BCM50. You can get the same rates running through the Cisco ATA device as well. You can make as many calls as your bandwidth will allow.
You will have to get a rather large ATA for 20 trunks.....or you could buy 10 PAP2 devices from Cisco and link them together for Vonage or VOIP.MS.
Vonage seemed geared towards residental applications and was rather weak and overpriced in my opinion.