If a router has port 5060 open, and I know that there is unencrypted SIP traffic going through this port, how could one take advantage of this?

port 5060 is normally assigned to SIP traffic. It might or might not be used for SIP however. A simple nmap scan to this destination should probably reveal much more, for example here's an output from a OS fingerprint nmap scan to a voip adapterSome implementations of SIP TLS appear to use port 5061 by default, but the reverse is not necessarily true. i.e. seeing port 5061 doesn't necessarily mean it's encrypted. I know of a few SIP installations where various ports are used for (standard) SIP, and they  tend to range between 5060-5070... Again, those ports are completely arbitrary. You can choose to run a service on pretty much any port you'd like. So I can, e.g. run SIP TLS on port 80 and plain SIP on port 23 if I choose to... Until you do some kind of a probe / scan, you won't be able to know with a high-enough degree of certainty.

• As far as VOIP / SIP security - there are probably many tools for scanning and potentially exploiting VOIP. A simple search revealed those items:
  
• http://www.voipsa.org/Resources/tools.php
  
• http://www.hackingvoip.com/sec_tools.html
  
• http://blog.sipvicious.org/
  

and I'm sure you can find many others to experiment with.

How can you tell if port 5060 is open? nmap -v -sV localhost -p 5060 will tell you the truth. You can also use: netstat -apnt | grep 5060 or ss -aln