If your phone has only one unique phone number, it is a single-line phone. If there are at least two unique numbers on your phone's line keys, you have a [multiple-line phone|MITvoip Telephone Multiple-Line Phones.
Regardless of how many phone numbers you have, a number might be assigned to more than one line key on your phone. Multiple line appearances allow you to manage multiple calls on the same line. If a phone number is a shared line, its phone icon will be half gray instead of black.