Sure, you may roll your own anything, but you are better off outsourcing much of that to cloud providers today. Freeing you to focus on your application that provides value to your users, not worrying about how to properly architect and scale telephony solutions.
Apache's free and hardware's cheap. Yet Rackspace still manages to run a huge web hosting business.
For some people and some uses, building a web server and hosting it on your home or office network is acceptable. But those that don't count "web hosting" as their core business and want the reliability, security, and stability that comes from having a professional do it tend to hire a company to do it for them.
Like most other things in technology, it's a simple build-vs-buy question. Is it more cost effective and better suited to my needs to build it myself or to buy from someone who's an expert?
Freeswitch and Asterisk are great. I was the emcee at the Freeswitch developer conference last year and we financially support Adhearsion, an Asterisk development framwork. They fill a great niche for DIY telephony. For everyone else, there's hosted services.
You dont need a bunch of servers either. If you are doing more than 200 simultaneous calls ( freeswitch easily achieves this on 1 box ) you shouldnt be looking at these entry level systems; Voxeo, Tropo's parent company has an incredibly powerful platform.