Yes, it is possible. You can create a record in your system and implement a pager/intercom to execute auto"magically" at some hour and every call phone will auto-answer and hear the messages -only apply to IP-Phones-
Depends the price and the plan from your provider, in my country is cheaper a VoIP carrier than land line or E1.
A server for run asterisk, if you will use E1 lines or analog lines a TDM card -openvox,digium or sangoma-, IP-Phones. This just for start.
Depends on your server features, per example: CPU Dual Core 1.8ghz, 1GB DDR2 667 Mhz, can support 40 simultaneous calls
Yes, but even depends on the CPU performance. When you generate a call this call use a codec, generally G711, but you can define your preferred codec like G729, this codec is more used when you use a Internet route to make your call. Every codec has a bandwidth consumption so if you have a internet dedicated of 2Mb you can generate 63 simultaneous call.
This is by: (2048 - 3% ) / 31.2, the 3% is the headers more or less that you consume in every call, and the 31.2 is the consumption for a call with g729 codec. Obviously you need a SIP provider.
When we talk about T1, here the maximums calls you can generate are the maximums channels in your T1 for the DSL applies the above answer. The CPU help to translate between codecs, so the performance will decreased if you use differents codecs in your call.
No, only one dynamic or static IP, but one sip provider.
Yes, you will need 40 differents land lines, for that number is better a E1,T1 or sip provider.