DSL service can be confusing. It's because DSL technology piggy-backs on the same telephone line used for voice telephone service supplied by your telephone company. Traditionally, people had "plain old telephone service" or POTS, and then they added DSL on top of it. Telcos were reluctant to sell DSL without also selling a POTS telephone number and telephone service, too.
Later, most telcos started offering "naked" DSL. This means: you still have wires coming into your house from that telco, but the dial tone and telephone number are turned off. The wires are acting only as a data line, into a DSL modem. It's up to you to contact your telco to find out if they offer DSL without voice telephone service, and to insist that they check into it further if they initially give you the brush-off.
OBi devices use Voice over IP (VoIP) technology, to make and receive calls over internet data connections. So, if your telco offers naked DSL, then yes, you can dump their POTS service and just use VoIP over your DSL service.
Keep in mind, however, that depending on the VoIP service provider you are using with your OBi, it may or may not support emergency (911) calling. Google Voice doesn't. SIP VoIP service providers generally do, often for a separate small fee. You need some way to call 911. Don't cancel your POTS landline service until you have some other means of calling 911.