A two or three line phone is like having three diferent phones, but just one handset.
They have two or three conections on the reat, for line 1 line and line 3.
Normally , they have hold, that way you could put one line on hold and answer another call.
If you use a external VOIP box( ATA 186, quintum gateway or whatever), you could connect the PSTN line to one port and the VOIP line on another.
You could make only one call at the time, but you do not have two or three phones on the desk.
An added bonus is the posibility to make conference calls from VOIP to PSTN trought the phone.
Levels normally are not too good that way (too much loss), but for a free international call, the price is right.
The most common multiline phone is the Panasonic, with a two and three line sets.
Not the cheapest for sure. Go to ebay and make a search.