I recommend you start designing your plan for planning. My recommendations:
1.- Who in your organization can provide you information related to statistics that your switch is already producing? get information, manual, etc)
2.- Who in your oganization is in charge of generating or polling the statistics from switch? get one file.
3.- Start looking for traffic in trunks and routes
4.- Try to calculate busy hour traffic, do a model in order to determine the max utilization (85, 90, 100 no recommended), make trends
5. Try to get the telco cost for circuits
6. Try to get if there is special promotions for traffic like valentines day
7.- Take a decisíon about your network, reduce, increment, everything is ok
8.- Do a revision plan to check utilization
Hope it helps
Regards