It will depend. I use my personal phone to access my company email, but we have Outlook Web App access since we techs were issued netbooks for doing our paperwork and email and such (never use mine, just do it on my phone). I am only connected through a web page so they have no access to my device.
My work phone, however, is completely accessible to work as it uses an email client and work has installed software that allows them to control the device if they so want. It is an iPhone so they are completely welcome to it (Android guy at heart).