Years ago I tried configuring Asterisk and was defeated. Well, not completely defeated as I got it "working," but I couldn't claim to truly understand the configuration.
Asterisk's configuration system isn't the worst thing I've seen, but it reminds me of Sendmail, which (if you don't know) is not a polite thing to say.
Of course now there are probably glossy Asterisk distros you can run in a VM and configure via WUI. But I'm obstinate enough that I want to understand the text config.
There is also FreePBX, but I know nothing about it.
Even further off topic, I'd recommend the Obihai boxes as a cheap way to get started. No affiliation, I just have an OBi110. It's about as easy to configure as Asterisk itself. (Why is all this VoIP kit horrible to set up?) However, Obi has a through-the-web quick configuration that will hook it up to Google Voice with no other hardware/software. That alone is worth the price of the box (which occasionally goes on sale). ~US$40 flat and Google Voice gets you free phone calls in the USA.