I am curious to the possible configs. If I strictly setup say Cisco call manager for all calling local, site to site and have the ability to call to the PSTN, I would have to contact the Telco to get outside access right? Like my VoIPs gateway would be Sprint or so? I am unclear as to how it all works still.
use the high density gateway cards in a Cat 6k switch - this tends to be the cheapest way to provide several T1 or E1 interfaces, esp. if you have the Cat 6k anyway...
public VoIP and / or SIP interfaces may be different, but interworking with H.323 between Call manager and other PBXes has traditionally been easy in theory, and painful / difficult / hard to debug in practice....
simplest way is 1 or more cards + DSPs in a cisco router - for just analog you could use a 1751 / 1760.
if you need just 1 or 2 PRI on T1 or E1 then you need a 2600 or 2800 series.
i havent done the price comparisons recently, but once you get to 4 or more the Cat 6k cards get to be cheaper.
note that the cards or the rotuer ends up with a bunch of DSPs to handle the codec functions, so it tends to cost a fair bit more than the base router.
if you go this way then you need to get someone to spec the router for you - the codecs needed depend on the number of active channels and the codec processing load. You can also use the DSPs for other jobs in some configs, such as transcoding or conferencing, so that should be taken into account.