The main feature of the PokéGear is the phone. The phone is used just as you would think and can call a variety of characters within the game. These characters include trainers dotted around Johto as well as vital characters in the story such as your Mom or Professor Elm.
With the phone, you can both call-up and receive calls from these characters. They will give you vital information when you call or they call you and some will allow you to call for rematches or to obtain items.
If you have an appointment to rematch or to pick up an item, the map will display an exclamation mark in the area when highlighted.
Unlike in Gold & Silver, you don't have a limit to how many phone numbers you can register in the PokéGear so you have the ability to get all the numbers.