I think you need to get clarity on agents getting kicked out of vicidial and calls dropping. These can be very different things. Agents getting kicked out often has to do with time issues either on the server or stamps in the db. Accurate clock is also very important in Vicidial. You can use pseudo clock, but we have seen issues with it. Have you got a telephony card or a
VoiceTime? There is also a difference between a call dropping and not being able to make a call. First generally points to a connectivity problem the second to concurrent call capacity. Call drops could be at various points, Agent, your LAN, server, SIP provider issues.
Vicidial is primarily aimed at outbound dialing. Are you dialing manually or using some of the automation features to queue up leads? This will have an impact on required concurrent calls you need to be able to make. You have not mentioned how many agents you have or how many concurrent calls you are paying your SIP provider for. I assume the diginet line is dedicated for voice only?
As mentioned by vince0 Asterisk creates a channel per leg of the call and then bridges them, so that's one channel to the agent and another over the SIP provider. For simplicity we rather refer to concurrent calls when speaking to the SIP provider so it removes the ambiguity. Vicidial behaves differently to a normal asterisk call in that it uses conferences to join parties together (which is why clock is important). Agents sit and wait in the conference, when Vicidial gets a lead to answer it adds that person to the conference. In auto-dialing mode it will then start calling the next lead to line them up for an agent.
The amount of concurrent calls you can carry on the line will be determined by the codec you use. G711 needs 64k, G729 only 8k. Any respectable SIP provider should offer G729. You would however need to enable G729 on your server. There is an unlicensed version available, but if you are running this in production then you should
purchase your license from Digium.
On the face of it your server specs look fine, depending on how many agents you have and what else you are running on it.
We have extensive experience with Vicidial, feel free to drop me a PM if you want to engage in formal support.
Edit: I was still typing this up when you posted the agent count. If all that server is doing in Vicidial for 20 agents, then there shouldn't be an issue with hardware.