First go and create your phone models, what is a very simple and straight forward database entry.
Once this is done, you create phones based asset TAGs. If you don’t want to put an asset tag sticker on e.g., a Cell Phone – simply enter e.g., the MAC address or the SN of the phone instead, something that is unique and helps you identify the phone physically.
Phones have a bunch of fields, not all apply to every kind of phone. E.g., will a VoIP phone not have an IMEI number, or a smart phone not necessarily have a fixed IP address. If you have an analog phone, you won’t have an IP address nor an IMEI or MAC address at all.
The fields allow you to handle all kinds of phones and help you identify as clearly as possible as well as document them for e.g., the purpose of handing out cell phones to employees and making sure you know what they have when they e.g., terminate / resign.