I'm also very interested in doing this (except I want to use Zabbix, not PRTG). I've poked around the SNMP MIBs hoping to find the number of SIP trunk licenses purchased as well as how many are used on each trunk, but I'm pretty new to looking at that information and have been unable to find anything. I don't know if it's because I'm so inexperienced that I'm not looking in the right place or if it just doesn't exist via SNMP.
In my case, getting the information from the provider would only tell half the story. In addition to the SIP trunk for access to the PSTN, I also have an internal SIP trunk to my Lync server. I really want to keep tabs on my SIP trunk usage because (I believe) an external caller connecting to a Lync conference call will use 2 SIP licenses.