Actually, you don't need any special junction box.
Put a fax modem card into your computer.
Get DSL service on your 2nd tel line. That single DSL telephone number is going to cover everything you need.
Have a printer with an integrated flatbed scanner.
When Verizon supplied my DSL modem, it came with four "Y" splitters. I didn't need to buy them extra. This is my only experience with it, but I'd assume other ISPs probably supply it the same way, and includie some "Y" splitters with the DSL modem they send to you.
The simple "Y" has a "filter" (resistor) on one lead of the "Y" intended to carry a telephone signal. Plug that side into the fax card jack on back of your computer.
In Windows XP Pro go Start / Control Panel / Add or Remove Programs / Add-Remove Windows Components / check "Fax Services" to enable Windows' built-in fax.
The other side of the "Y" splitter goes to your DSL modem, then to your router & LAN port on back of your computer.
If you install a VOIP softphone in your computer, your telephone is carried on that same line as the DSL line.
Summary: All you need is
* DSL service, with DSL modem & simple "Y" splitter, several of which may already be supplied by your ISP with the modem.
* Fax card
* Enable Windows XP's native fax capability
* Printer / scanner (flatbed is vastly better)
* VOIP
When necessary, just use your scanner to image what you want to fax out.
That works real well for me, and is how I have it set up.
ALTERNATIVE: Instead of putting a fax modem card inside your computer, you could just connect the resistor "filter side of the "Y" splitter directly to your standalone fax machine's input. That'll give you fax, & if that has a phone handset built into it, there's your telephone too. Simple as pie! The only thing is, incoming faxes will be on paper instead of digital in your computer.