Forward an ISDN call over wifi

I have one house and two warehouses a few meters apart (different landline connections and different numbers )

Now I pay 3 different phone bills for my three different numbers.

But since I dont use the phones on the warehouses that much what I want to do is

Create an ISDN subscription on my house with 3 numbers (one main and the others as MSN ) and forward as VoIP over wifi the calls from the other two MSN numbers to the warehouses that I keep nearby my house.

Is it possible ?

What kind of gear do i need to accomplish that ?