Good For you
[quote] We are still working on the details of the configuration but a question came up about where to place the asterisk server.
We have co-lo space with 100Mbit connections, fully redundant network, power etc. [/quote]
If you had the Budget, i would have suggested one in the office and one in the Co-lo, the one in the office you run as a thin Client server, the one in the Co-Lo you use for the routing of all your calls.
You would need two Asterisk servers, have them speak to each other over IAX2 (I find it more reliable then SIP to be honest), and you can then manage to do Internal Lease Cost Routing I.E the Office Asterisk Box handles inbound calls if you want to retain the PSTN side of things, and outbound can be routed through to the Data center. Generally speaking the Bandwidth supplied at Data Centers is much better, it is usually non-contended if you ask for it, and has better reliability then the standard ADSL services you get in your home or business.
Your upload is not that high, it is a Pity as you may have to restrict yourself to using G.729 CoDec, unless of course you manage to keep it with as little as possible with the amount of concurrent calls going at any given time.
Then make sure you have fail safes or redundancies in place, having the two servers can actually be of great assistance, how can be based on your other comment.
By having the two servers, one in the office and the other in the Data Center, you can have your Toll Free route to the Data Center and then send all traffic to your Asterisk box in the Office, if for some reason your Office DSL service was to go down, you can at least have two options to play with, 1. You can leave an Announcement on your Asterisk Box in the Server, possibly even voice mail, or, 2. Whilst the Asterisk box in the Co-Lo is busy talking to your customer when they call up, you can be organizing a dynamic re-route to your office asterisk box over the PSTN lines.
In any event i strongly suggest you have at least 1-2 PSTN lines still in service (ISDN-2) going Full VoIP is still not a fantastic Idea especially if your Business is mission critical on phone systems.
I would suggest though in any case, that you make sure you take your time with it, sit down and map it out on paper first, and possible even get a network consultant in to help with the role out, unless of course your a network engineer yourself which you wont need one then