You can use multiple phones on one  BestVoIPUSA line

Multiple Telephone Adaptors       

Use just one adaptor to power your cordless system, or use multiple adaptors! The benefits of adding another adaptor are endless. You can now make more than one phone call at the same time. As you add adaptors you are in turn adding another "line" to your household. It carries the same phone number, and will ring just as all the others do! You can also call to these adaptors individually from anywhere in your home. For example: Your master bedroom has an adaptor labeled 100. The Family room is using an adaptor labeled 101. You can call directly to the master bedroom by dialing its extension. The possibilities of VOIP are endless!

A Cordless Phone System
 

Some cordless phone systems come with base units and the option for extra handsets. The base station of the cordless telephone plugs directly into the Digital Telephone Adapter and multiple handsets can be placed anywhere in your house. The only limitation to the number of handsets you can have, depends on the number of handsets your unit supports.

Using Feature Phones

Feature phones allow you to truly utilize all of VOIP's amazing technology. Feature phones give you the ability to place call after call, while putting the other party on hold. Conference, speed dial, transfers, and multiple lines can all be processed on a feature phone.