Hi Aaron, Sorry you're having data usage troubles, I know how frustrating that is in situations where you have a limited amount of data allotted. You mention that your data is wireless, are you using a cellular or satellite carrier for your location?
Most VoIP phones use about 100 kbps/call with modern codecs like G.711 or G.722, which comes out to around 6Mb per hour for one call. This is a very low data usage compared to other internet traffic for a comparable time, such as browsing websites. If your network equipment supports it, could you monitor the data usage of devices on your network to understand where the high data usage is coming from? Many routers have the ability to monitor each device on the network according to its MAC address, then give you running totals of how much data they are using over a period of time.
Let us know if you are able to monitor your Cisco phone's usage and what amounts they are using.