This is becoming a problem for me, because I'm taking most of my calls on my mobile these days as redirects, but I think I'm just about to nail it. I have a landline, a VOIP line and a s60 smartphone. I also have skype numbers and non-geo, but they're unaffected by this scenario.
I'm about to install a full-time Voip switchboard, which will take incoming calls and route them them to both my land and voip lines. If I set up my mobile phone with SIP software, I can use my mobile as a numbered extension.
Because i work at home, this means I can route personal calls through it without getting disturbed.
The downside is that it means yet another PC on full-time, but if I stick an energy-efficient PSU into it, it will only suck under 100 watts, and I'll just have to plant some more trees.
The good thing is that I can then route calls to all the PCs on the network, which at the moment is three in constant use and one other occasionally, and any new S60 smartphones we get.
We've recently become authorised to install and train for the 3CX Windows PABX switchboard software, and have done some project management for Voip installations so I'm looking forward to setting it all up.