The standard redial button enables the basic redial function. This simple implementation only stores one number to redial. Every time a call is placed, the phone saves the last number dialed to this memory. Previous contents are overwritten. Pressing the redial button will call up this memory and automatically dial the number. This only allows the last number called to be redialed. It can be redialed until replaced by a different number called.
Modern phones, cell phones and smartphones with display typically feature advanced redial functions. This allows access to any number from a large redial memory. So you may access one of the last ten (or more) numbers dialed at the push of a button.
Even if a phone doesn’t have a redial button, it may possibly redial. After all, this function can not only be provided on the devices themselves, but also via switchboards or telephone systems. In this case a central server stores the last number dialed from a phone. A defined key sequence can then be used on a device to redial.