@Leliana: I believe OP is referring to Asterisk the VoIP software, and not the asterisk symbol (i.e. wildcard certificates). At least I hope so, otherwise the rest of my reply will be completely off-topic.
I skimmed through the Asterisk wiki page on this topic, and as far as I can tell, there's no reason why it wouldn't work.
tlscertfile expects a file with both the certificate and your private key, which you would have to generate yourself if you're using the official client (simply by concatenating
privkey.pem), though that will probably be solved in future updates.
All currently available authorization mechanisms will require you to run some kind of web server to serve challenge files on the domain you want to generate certificates for. The client has a standalone mode which will spawn a web server and serve the challenge files for you, but you have to make sure it's available from outside your network. In the future, DNS-based challenges will be available as well.