Iron hit the nail right on the head with his post.
However i make comment though on the Codec selection.
Currently in our Setup we have Dual Xeon 3.2GHz Servers with the latest Intel processors, and i can tell you with this CoDec even with such power it chews the ass out of the CPU, the difference is that with the latest Xeons you don’t loose voice quality until you reach about 97% usage of that CPU. So in saying that best of to avoid this CoDec unless you absolutely need to use it.
These are the better of the CoDec choices, however whilst they do very little damage to CPU load, they do however chew the ass out of the bandwidth, and once your link reach 85%-90% Saturation they may produce really choppy voice quality, even with a non contended 1-1 link (Which is what we use) this may still happen and most likely wont be avoided.
If your not thinking of doing any faxing over the data link and are most likely going to have a lonely old POTS line handling the fax, then i would also recommend the G.726 CoDec, whilst we are still running testing on it and the Jury is still out, this CoDec has so far shown it can really preform and in most cases it has even equaled uLAW/aLAW voice capabilities. However we personally ourselves are having a damn hard time getting our Tier-1 Carriers to use that CoDec, they just don’t like to play ball, every time we ask they keep telling us to only use G.723 or G.729 which i am not a big fan of myself.
Now i don’t know exactly what costs are for Bandwidth in the states, but if you can and if it is possible, try your best to get a non-contended 1-1 service or if not try and make sure it’s contention ratio is not greater then say 20-1 this way you can be sure that you can achieve best quality results of the data packets.
Also keep in mind, that no matter what you should have at least 2-4 POTS lines spare if you can, phones with businesses are mission critical, and generally pots lines do not fail, where as Internet Links have so many points of failures they still are not reliable enough to run your business of it solely (This can be argued, but it is just my opinion), also what ever DID’s you get, make sure the ITSP can do re-routing of calls to those DID’s to some back up lines, either the spare POTS lines or something else, this way your ass is covered.
Hope this helps.