Card Selection

I'm getting ready to build a new box and this is my first project. Currently I'm at a point to pick a telephony equipment but first, I'd like to know the difference in between digital telephony and analog telephony. I have a basic understanding of it. If the telephone carrier who brought the lines to the premises used a T1 box, then I'd assume I have T1 and I can go with digital telephony equipment. Or, if they brought the lines in an old fashion way then my only option would be to go with analog telephony equipment. Is this it?

Digital telephony equipment seems to be superior in terms of flexibility. One small card can handle up to 30 calls, not to mention a dual port one. Analog would require a humongous card with 24 lines only. But how reliable is digital telephony? Does the service go down pretty often like cable modem internet or DSL based internet would go down? I'd assume analog phone lines wouldn't go down that often since I know that even when the electricity goes out, you still have the good old dial tone with analog lines. Also, which one has better sound quality and less of a echo problem?

I appreciate your input. Thanks!