A friend now has a second number allocated by BT which is prefixed by 056, Which I understand is a Voip service.

If I dialled it from a Post Office Homephone account how much would I pay for the call i.e. what is the tarriff?

I have tried the Ofcom website and asking Homephone without success. I understand that BT would set the cost. Cheers

PS I suspect I might pay a similar charge to e.g. 0845 or 0871 numbers.

Hi. The BB Talk number (056) is a national number is is charged at the normal rates that your service provider charges for these calls

National and local number pricing fell out ages ago though; it's just "UK Landline" or not. Does calling an 056 number come out of a mobile's free minutes, or any inclusive call package on a landline?

They don't come out of mobile inclusive minutes, they are charged - and at a higher rate than calls to a landline number.

However - if that number is on a BT home hub, there is a local number assigned to it as well (in this case at least). The owner of the hub needs to log onto it. I can't remember the exact menu but I came across it when making some changes to a friend's hub, I spotted a local number listed in there and called it. It rang the hub phone.
Outgoing calls from the hub always show up as the '056' number though.

A customer will only get allocated a geograpical number (ie same exchange number as the BB host number) if asked for at the point of ordering, if not you will be allocated a 056 prefix number.
If you have a 056 number you will not have a local number as an alternative, if you go into the setup of the latter HHs it will give you the BB talk number.
O/G calls from the hubphone will show as the allocated BB talk number unless you prefix the called number with a 5, in this case it will display the main pstn number given to the line.

The hub that I looked at must be a one off then as they certainly haven't ordered a hub phone with a local number attached.