The only thing that is wrong with the wording in this question is... The wording in this question. Unless you're confusing the issue with say a Cisco router running Call Manager on it. Routers route and are not PBXs and those that are primarily fall into VoIP. Even PSTN based routers usually transcod into VoIP via FXO or FXS cards depending on what their goal is.
Remember, routers route - PBXs make calls. It is seldom seen that you will find an ingress router at the campus or branch edge which does both, act as the corporate router and, the corporate PBX. Let alone finding a publicly accessible router/CME or cube at any of those locations. You *may* stumble across one, but the likelihood is low and the engineer who placed it there is off his rocker.
In either event, you'd want to familiarize yourself with routing as a whole and VoIP on a generalized scale. Start with Cisco's SAFE (http://www.cisco.com/en/US/netsol/ns954/index.html
) to understand design, configuration, security and implementation. For SIP slash VoIP certification look @ the SSCA from SIPSchool (http://www.thesipschool.com/courses/view
) which is recognized by most IP PBX vendors. Other than that, the rest is vendor specific (CCSP + CCVP, Acme's ACPS/ACPN and so on)