Can someone take advantage of the router that has its port 5060 open?

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 adapter

nmap -v -O 
...
Host is up (0.0026s latency).
Not shown: 999 closed ports
PORT   STATE SERVICE
80/tcp open  http
MAC Address: 00:0E:08:CA:**:** (Cisco Linksys)
Device type: VoIP adapter
Running: Sipura embedded
OS details: Sipura SPA-1001 or SPA-3000 VoIP adapter
Network Distance: 1 hop

TCP Sequence Prediction: Difficulty=261 (Good luck!)
IP ID Sequence Generation: Incremental

Some 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.