Yes.  While many appreciate the convenience of having one phone for work and personal use, here are the risks of doing so:

• Employers Listen.  The Electronic Communications Privacy Act allows employers to listen in on calls.  Under this law, employers are not supposed to record or listen to private conversations, but this does not provide much reassurance as to privacy.  Specifically, employers are permitted to monitor calls until they determine that the communication is of a personal nature.  By the time this determination is made, it is likely that personal information will have already been divulged.
• You’re being Tracked.  Cell providers give employers access to information that identifies the individuals to whom and from whom calls and texts are made, and the date/time of each interaction.

If you choose to use a work cell phone for personal matters, do so with the knowledge that your communications may be monitored.