The traditional and hard way is to start as a first-tier NOC person for an ISP and work one's way up. Or to start as a field tech for an ISP and show skills worthy of getting promoted (perhaps into a NOC job or a manager-of-field-techs job). That method takes quite a while.

Medium to large sized ISPs have a very large amount of BSD/GPL/Apache/misc licensed software running to support back end monitoring and provisioning systems. They do occasionally hire software developers to customize things for their environments, so it certainly wouldn't hurt to reach out to the noteworthy ones in your region and try giving them your CV.