Hey Mike! Welcome.
My suggestion, start at the bottom dude, it sucks but it's a start. By that I mean start at layer 1, your physical connections, learn how to wire jacks for single line phones, RJ-45 connections, pin layouts for T-1 lines, RS-232 connections and so on. Some of this is no longer used but your understanding of how things work and communicate will expand greatly and make you that much more valuable.
A couple things, POTS (Plain Old Telephone Service) line, this is just a telephone line from your telco (telephone company). You can start moving into the layer 2 as well, learning about protocols, networking, switches, hubs and much more.
Look at your questions this way for now, a PRI T-1 comes from your telco and into your network. Than you have phones or extensions that use that PRI to make incoming/outgoing calls on. You can do this on a POTS as well but a POTS is just one phone line, not 23 like a T-1 PRI.
SRST-in short is a survivability term so if you lose a connection from a router back to your phone system or Call Managers (referring to Cisco here), the router takes over and keeps a certain number of phones working (depending on your router size will determine how many phones can still function). Than on the router you might have a T-1 or just a few FXO (or pots lines) for incoming and outgoing. In srst mode you dont' have the full functionality as if connected all the way back but enough to answer/make calls and transfer until the connection is restored.
DID's are basically direct lines to an extension, a voicemail or a hunt group. Say you order a T-1 line and need a block of 50 DID's, the phone company will assign you say xxx-5000 to xxx-5049 so than you can program your system to have xxx-5000 ring whatever extension and so on.
Cisco has moved to a Linux OS for their newer call managers, so obviously the more you can learn the better.
Linux would be great, but even better, become a genius with routers and switches as well. The actual voice side is not too hard to get, once you learn the terms and how trunk groups, hunt groups, pilot numbers, routing patterns, calling search spaces, partitions, and so on interact and function with everything else.
Any other questions just let me know and I'll try to help you out! Good luck.
