Connecting a phone to internet router (B-Box 2)

Dear telecoms and computer experts,
please help me since I have no clue about all this...

We only have an internet connection from Belgacom without separate phone. It has a router - B Box 2 -, which is VoIP-enabled I read.

We would now like to connect a phone on which we can also be called through the router (i.e. without going through PC), i.e. we would like to have a phone number.

How do we do this? What subscriptions and hardware is required? I saw that there are phones with Skype built-in. That would be one solutions, but seems there are also other ways to use VoIP.

Thanks for your help!