Hi bt hub phone fits onto your bt hub you have to remove the plastic panel at the bottom of the front face and the hob phone base clicks in there.
As for calls it will depend what package you are on our is free calls for up59 mins at evening and weekends.
it has its own seperate number so you can teat it like to lines we use our main line for our fax machine and the hub phone for calls
hope this has been of some help
best of luck
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