i think i wanted to know more or less if anyone does this in a production environment as well. this came up with a customer, he didnt want to buy both call managers but only one. so i was thinking to offer him max redundancy i could have SRST on the local router. then it came up, suppose you have 300 users? we dont have a router to handle that with SRST so we would then have to pick and choose which phones should register and which cannot....

With single CM server, SRST as backup would be better than nothing. I cannot speak to best practices, as I would never implement solution with a single server, 300 user company is large enough to afford second server.

As to the selective SRST registration that would be your only option if the router is a 28XX type of a GW. So, you would need to pick the most critical phones, the challenge with that is with time as people come and go this gets lost and the next thing you know is that someone important was unable to receive a call while in SRST :-(

Chris