The term "digital" used there is ambiguous. For example, you can plug in a cordless phone base station, and use its handsets. Cordless phones use DECT, which is a digital wireless protocol, to communicate between the handset(s) and the base station. But the connection between the base station and the OBi device is an analog telephone line cord.
The other, completely different meaning, is a IP phone. These phones connect to your home or office network (LAN), instead of to an analog telephone line.
Any modern DECT cordless phone should provide decent sound quality. I recommend Panasonic models. If you want a desk phone, then the OBi 2182 IP phone offers very high sound quality, either via its handset or speakerphone.