On iPhone, with an iPhone data contract, if you are in poor coverage switching between "No Service" and some coverage, the "System" will start using a fair amount of data communicating with Visual Voicemail.
You can see it happening sometimes as the phone app icon will show a red blow when VisualVoicemail is "errored" and blob will disappear when coverage comes back.
You can check if this is you issue by going into Settings, selecting Mobile, and down near the botton is a sub-section for System Services which when you select will show you how much different services are using (including Visual Voicemail). If it's not visual voicemail then this display will show you what is consuming your data. However, this is only useful is you reset your stats (in the same screen) otherwise it will just be the accumulated data you've ever used !
I have a reminder to reset the Mobile Data stats at the start of each month.
As I have very poor/intermittent coverage at home, I previously switched my data contract from an iPhone contract to a generic data contract - so I gave-up visual voicemail and the issue disappeared. I've since got in the habit of switching to airplane mode at home to preserve battery life a bit (as low signal/no signal is a battery killer).
