I understand an employer can track my Company issued cell phone with a GPS device. My question is- can the employer listen to my conversations

Probably not, BUT................. You should understand that any company owned cell phone, computer or other "electronic device" is the property of the company.  Therefore, anything transmitted or received using the equipment is subject to monitoring by the company and you should have no expectation of privacy when you use it.  From a technological standpoint, it would be difficult for the company to "listen in" on your phone conversations, but it can certainly see and monitor and see any text messages, e-mails, photos, "search histories", etc. which are transmitted or received using the device.