Most likely your problem isn't the cable, it's the way it's terminated. Gigabit requires all 4 pairs. Some sloppy folks got in the habit of only using two pair of wires, since this is all that was required by 10/100. Also, I've seen lots of newbies incorrectly wire their cables 1, 2, 3, 4 rather than 1, 2, 3, 6. If you know how to terminate Ethernet, this will make sense to you. If you don't understand what I'm saying, do some reading:
If it's wired 1, 2, 3, 4... you'll have errors even at 100. 10 will work fine.
Anyway, I'll state it again, I'm betting all 4 pairs are not cabled through. Your NIC and switch will automatically fall back to 100 if they aren't wired right.
Oh, and if they were wired for phones, there's a chance they used CAT3, which will only have 2 pairs. You can do 10/100 on CAT3. I haven't seen CAT3 used since the early 90's, but it's possible...