The thing is, I no longer have a house phone. I just want to hear my mobile ring when I'm (for instance) in the bedroom and I've left my mobile lying about in the den. This has turned out to be the one major drawback of finally giving up the old landline.
I've seen a few do-it-yourself kits to build a ring detector based upon RF detection from the phone, but these have very short range. One guy made a "dock" for his phone and used a photocell to detect incoming calls when his phone's screen lights up. I'm surprised that nobody is marketing a device that could ring a standard doorbell when you have an incoming call. It would be convenient if there was a docking device that also doubled as a charging station for the phone.
I'm accustomed to always having my mobile nearby when I'm away from home, but I'd like to be able to just put it down somewhere when I'm in the house without worrying about missing a call. Monitoring a computer display isn't a solution. If I'm going to do that, I can just as well monitor the phone.