The 100 is for when you want a phone and don't have a phone line from TPC (the phone company). It has slots for 2 VOIP providers.
The 110 connects to a single phone line and also includes 2 VOIP providers.
The 202 has no connection to TPC but has slots for 4 VOIP providers and 2 phones. This allows options such as calls from a particular provider can go to one, the other or both phones, while another provider might only ring phone 2. Each line can operate independently.
So if you need to connect to a physical phone line, the 110 is the only one of the 3.