Every VoIP account comes with unlimited lines, so you simply don't need to worry. Unlimited basically means that you can have as many outbound or inbound calls as you wish on the account.
Lines are different from Users. Each User can have up to two handsets registered, as well as a softphone on a mobile, and a desktop/browser app connection. These will all run concurrently, and unless set to Do not Disturb, any inbound calls to your User will ring on all registered devices.
If you need any further help today, please don't hesitate to contact our friendly support team on 020 8050 5141 or by email!