hello again! I assume from your post that you are planning on dumping the AltiGen gear and going to a straight asterisk setup. So here is another somewhat useless and very long rambly reply.
When they say only one Digium board, that generally means not more than one quad-PRI board (~100 channels). Unless your server motherboard sucks, you won’t have IRQ issues, and as long as each card has its own IRQ to itself youw ill be fine.
So you have 24ch PRI -> 36users, and 5POTS -> 4users. I am also assuming that all your users use analog (ughk) telephones. How are they connected? I’m assuming they are plugged into interface boards on the AltiGen system…
Since you are diving into VoIP, you have somewhat of a decision to make- if you are going to transition to a mostly voip-based infrastructure, now is the time.
FXS interfaces for analog phones will be at least about $70/port no matter how you slice it, and that assumes everybody has a phone and wiring.
IP phones are a bit more expensive, depending on what you get they start at around $100/phone(port) ($80 for a sipura-841 which is quite capable) (assuming you have adequate ethernet infrastructure), however over time and as you expand, they may save you money in lower hardware requirements (with reinvites, you can put many more than 100 IP phones on a server, so you don’t need a second box for quite a while). Also, IP to IP calls don’t require echo cancellation on the server, in fact they don’t require much of anything unless you’re talking on an outside line or recording something (because if allowed to do so, IP phones will send the voice data directly to each other; only using the server to set up the call).
Also- what kind of phones do you have now? Your AltiGen analog phones may not work correctly with a standard analog channel, in which case they will be useless and you have to replace them anyway.
My preference would be to (if you can sell it to those holding the credit cards) give everybody a VoIP phone. This will drastically simplify your backend needs, and with it you will be able to (unless your server sucks) expand to at least 100 users without needing any more servers or hardware.
If the Ethernet infrastructure isn’t there (which can be worked around by getting dual-port phones) and/or you think you aren’t ready for that, then dont bother; but if you think you will and you can afford it, now is the time.
That said, if you do go then analog route, you’d have to consider if you want to use PRI cards and channel banks or TDM2400 cards. Like I said, it comes out to be $70/port either way, and (assuming your mobo will correctly give each Digium 2400 card its own IRQ), i’d still go with 100channels/server max (although 4 cards/server is sort of pushing it).
As for dual-cpu boards- the advantage of this comes when you are crunching something. That means either encoding to a low-bitrate codec such as G.729 or iLBC, or echo cancellation for very large numbers of channels. IMHO, get one GOOD cpu and you will be fine.
As for multi-server, there are a few ways to set it up. How you do it will depends on what your calling patterns are, and how your company is organized.
If your users make a good quantity of internal phone calls, what will work well is to have a front and back system. Have one server with a bunch of TDM2400 cards that connect to your phones. Link it via a dedicated Ethernet span (TDMoE) or IAX trunk (both support more than 24 channels per span) to the other server. The other server will have your Dual- or Quad-PRI board and up to 100 channels of PRI T1 to your telco. This way loads such as echo suppression are balanced between the two systems.
If your users rarely call internally, then this system won’t save you much. For now it might be better to put everything in one server and then expand later as needed.
lastly, at least as an option- setup a system with FXS ports now for your existing users, but add voip phones for new users as you expand. This isn’t as efficient as going all-IP, but it greatly reduces your investment in old technology.