We do very little TDM stuff on larger (100+ phones) installations anymore, it's all SIP now and we as long as you have reliable connections to your ISP and/or SIP provider it is great and trouble free. Converted a lot of customers over from PRI or analog lines to SIP to. My suggestion is to pick a quality carrier over the cheapest ones though, it makes a difference.
For that matter, we rarely do even do physical equipment anymore... everything is virtualized now days.