Running your telephone over a CAT5 cable

One point one you are correct.  There are 2 'free' pair on 100 Mbit networking but gigabit would use all the pairs.  Reduced performance is certainly likely as RobWill mentioned.  The phone line is not too bad when not in use,  pretty much that same as POE (power over ethernet) but when the phone rings or is in use you would see reduced performance on the network.  That said it would likely be pretty low impact, possibly not noticable for regular home use, browsing email etc.  The 10/100 switch is a good idea though.

There really is no 'box' to do this right short of actually using IP phone technology.    Probably easier to use a cordless phone if you need top performance on your networking.  Depending on the layout of the house putting in some extra cables for phones may not be that difficult either.

Often people are sold 'network ready' homes that are wired this way.  Rather frustrating as doing it properly with 2 cables costs almost nothing in the overall home construction.

I agree, my first step would be to just try it out with the phone on the extra pairs.  It costs nothing and takes little effort but has a pretty good chance of working just fine.  Use a wall plate with jacks that are adjacent so you can keep the 'non-spec' impact to a minimum.

I would expect that even with some losses your networking would still outperform typical wireless networking by a large margin.  Thats assuming the wireless networking is actually working good.

If there is a noticable problem you really have not lost much and can go ahead with other options.  I would still probably run extra wires for either the phone or network or use cordless phones - whichever is easier or cheaper - rather that go with IP telephoney at home.

Voip or IP telephoney is just a lot more complex than a dialtone down a plain copper wire.  Running multiple extensions with IP equipment is not so simple as analog phones either.  

There are analog 'adapters' which provide FXO ports - which allow you to connect a phone line.  This converts the signal to IP but the one catch is that this is a point to point arrangement.  So a 1 FXO port adapater would allow for a single IP phone connection.  You can also use a FXS adapter which would take the IP phone signal back to analog to allow a regular old phone to be used.  Without a phone system or PBX you more or less just directly map the IP phone or FXS port directly to the FXO port.  

Cost is typically around $50-90 per port,  you can see a variety of different makes and models at the link below.  This requires no monthly service,  just the hardware.

http://www.voipsupply.com/index.php?cPath=96

There are also FXO/FXS gateways which are multiport versions - 2,4 or more ports.

To properly manage multiple phone sets and lines you would need a phone system.  Simple voip gateways dont have the abilty to manage call traffic.  So for example having all your phones ring on a incoming call would be a problem.    You might be able to trick a mult port FXO gateway by hooking all the ports to the same phone line.  Never tried it myself.

Though a PBX is more complex it is not necessarily all that expensive.   There are a few open source software PBX's that will run quite well on almost any old computer and are fairly simple to setup.  

http://www.trixbox.org/

http://www.asterisknow.org/

Any type of PBX is going to give you a ton of features, probably way more than you even want for home use.  If you actually want the extra features it can  be well worth the effort.  The downside is a computer is now needed to make or receive phone calls as compared to a simple copper wire with a dial tone.

Low end 'appliance' type IP PBX's are going to cost around $600 - 1200 range and up.  These have the benefit of being 'plug and play'.