Hey guys, a couple of things here. Voice channels are only 56 or 64K when they are digitized. But for the issue about replacing the 50-pair tie cable. You would need four strands of fiber and four channel banks having the appropriate interfaces.
All-in-all it would be cheaper and easier to replace the 50-pair with a new one that is at least Cat3.
Used to be. Nowadays it seems everything has gone from circuit switched to packet switched. But I don't know if that means that inexpensive used circuit switched gear is available. It may be more economical to replace the cable.
If your cable carried only voice, then you need something that can give you up to 50 voice channels. In the telcom industry, there's the standard D4 channel bank, which gives 24 voice channels over a 1.544 Mb/s DS1 connection (some makes include two systems in one chassis). You'll need these at both ends, plus some means of placing the DS1 signal(s) on the fibre. The lowest bandwidth I've worked with on fibre, is DS3, which is 28 DS1s. An alternative, would be some sort of voice over IP configuration.
Actually, FTTx technologies that use a single fiber are using two wavelenghts (colors) downstream (voice is being packetized and sent as data) and one upstream for data return. Then there is DWDM with uses like 12 or 16 wavelengths per strand. But I don't deal with that stuff so I don't know too much about it.
It's difficult to comment on the applicability of this solution as we don't have all the information. If you do a straight cost analysis it may be cheaper to call in a company to do a directional bore between the buildings and lay in a new 50-pair or even a 100-pair direct burial cable than to replace the existing run with fiber. Bells and whistles are what makes the world go round, but it all gets down to how much are you willing to pay for the finish on the bell or whistle? Spending $10,000 for a $2,000 problem doesn't endear you to the financial people.