We had this...
I worked for a UK national newspaper and we wanted wholly fault tolerant comms as we would scan and send our broadsheets to the printers. Basically it was a large fax machine but cost rather more than your average office Fax (think upwards of £500K each 20 years ago).
Anyway, we asked a couple of telecoms providers to come in, Cable and Wireless were one from memory (yes its that long ago), might have been BT for the other, we asked them to confirm they were independent, yes, we were told, one cable would go out the back of the building, one out the front (we weren't stupid, we'd worked that single point of failure out ourselves), We then asked them to physically walk where the cables then went. They got their maps and engineers in and we walked down City Road in London. One company went from the front of the building, the ones at the back walked round the perimeter of the office and lo and behold, they met at a junction box (or whatever you call these things in the ground) about 50M from the office.
Great amounts of embarrassment from each of them, we then had a more sensible discussion about where there cables were now going to go. I suppose eventually they met up somewhere but we couldn't find out where.
You just need to hold their feet to the fire and assume that the sales droids in the telecoms company's are idiots and will tell you absolutely anything for a sale. Once you know how they work, its fine.