Asterisk is in a way both networking, telecoms, and software.
There are opportunities to work with asterisk as a career and if you want to take is seriously you should have strong development skills.
If its just as a support or sideline to your overall IT career then good linux skills should be sufficient.
In order to be able to earn a living with asterisk, you have to be able to offer “value adds” around the asterisk software which as you must be aware is free under GPL.
“Value adds” may include development of GUI’s, integration with CRM or accounting systems or security systems. Pretty much what every you can imagine being valuable to someone that they would part with money for those services.
if you would like more advice contact me on charles --at-- inventigo dot co dot uk