A Device is what you will use to answer calls. It is usually a cell phone, but it could also be a landline telephone, such as one you have installed at home.
Whenever someone dials one of your business phone numbers and it is routed to you, Tresta will try to reach you at any of the devices you have configured on your User Profile. If you have more than one device set up, Tresta will try to call you at all of the devices simultaneously. Whichever one you answer at first will be connected to the caller.
Let's look at an example:
Jane works as a Salesperson at ABC Co. As a remote salesperson, she often works on the road or out of her home office. Her home office doesn't get great cell service, so she has a landline installed at her house. Whenever Jane gets a business call, she wants Tresta to try and reach her on both her cell phone and her house phone, so that she receives her calls no matter where she chooses to work. Jane has therefore setup both of these devices on her User Profile. Whenver someone calls Jane, Tresta will ring both of her devices at once and connect the call to the first one that is answered.
The caller will never know what device you are using - all they know is they dialed your business phone number, and you answered.