In my limited experience they are very flexible.
We had two phone lines to our house, originally via a telegraph pole in our garden. The pole actually belonged to the electricity company for an overhead supply to the house and it was all a bit unsightly. I arranged to have the electricity supply buried (which they very helpfully did for free, with me digging the trench) and a new meter installed in an outbuilding so that we didn't have one of those ugly white boxes attached to the house, leaving just the phone line on the pole.
I called BT and explained that the pole they were hitching a ride on was being removed by the electricity company (which was sort of true), hoping that this would mean I wouldn't have to pay for the phone line to be moved - and it worked!
Someone from BT came out to have a look and we discussed how to route the new phone line. I explained that I wanted the new line (from another pole much further away) to be fixed to an outbuilding, then routed along the fascia board, then to enter the outbuilding before exiting under about 10m of grass then going up the side of the house and then running into the attic where I wanted the two new master sockets to be fitted because that was where I had had routed a number of Cat5e cables from around the house to form a sort of 'patch panel'.
So, a far from standard installation but the BT chappie didn't bat an eyelid except to say that the section under the grass would have to run in a suitable conduit, which he would supply the next day for me to bury and then call him when I was ready. True to his word, the conduit (basically a 2-inch black pipe with fairly thick wall size) turned up the following day and I was able to dig a suitable trench and install it in my own good time. When it was ready, I called BT and they subsequently turned up to remove the existing phone line via the pole and install the new one along the routing we had discussed right up to the two new master sockets in the attic.
I have to say I was very impressed with how helpful they were - in fact, both BT and the electricity company were really helpful and the end result was no more unsightly telegraph pole in the garden with overhead wires to the house. And all for no charge!
I won't mention the subsequent fun and games over the following few years when the electricity company 'inspection teams' came out each year to check the overhead cables that no longer existed, or the meter readers that insisted on reading the meter that had been removed when the new one was installed. That would be another long story.