Re: how to make outgoing calls from land-line while on mobile
 

I think you can refer this thread for some suggestions.

An "adapter" to make and receive calls with a landline?
http://talk.maemo.org/showthread.php?t=68931

Re: how to make outgoing calls from land-line while on mobile
 

for the incoming line, from your mobile to your house, I would suggest you use a gsm adaptor with a sim on the same mobile company.

then the n900 to house leg of the call is liable to be free or at least cheap. then route it out the landline.

any free pbx software will do this.