Well originally they had no mains at all, just the 50VDC from the exchange batteries, but recently (last 20 years or less ) mains has been added to those that have had to support ADSL and G-Fast  equipment by getting the DNO to  run in a  line from the nearest substation, Now, as the fibre network is passive, power will not be needed, I suspect that most will not originally have had an adequate supply, or at least the fuse in the head will need changing. Equally  for mechanical reasons the DNO will have run in at least a lamp post level supply - which in many areas of the UK will mean it is 10-25mm2 even thought BT only needed 5 amps.
The bigger question will be the common one these days, of how much slack capacity is there on the DNOs transformers - round here fan kits have been popping up on substations and transformers outdoors in odd places, presumably as a far cheaper retrofit than a transformer swap.

Mike

Well originally they had no mains at all, just the 50VDC from the exchange batteries, but recently (last 20 years or less ) mains has been added to those that have had to support ADSL and G-Fast  equipment by getting the DNO to  run in a  line from the nearest substation, Now, as the fibre network is passive, power will not be needed, I suspect that most will not originally have had an adequate supply, or at least the fuse in the head will need changing. Equally  for mechanical reasons the DNO will have run in at least a lamp post level supply - which in many areas of the UK will mean it is 10-25mm2 even thought BT only needed 5 amps.
The bigger question will be the common one these days, of how much slack capacity is there on the DNOs transformers - round here fan kits have been popping up on substations and transformers outdoors in odd places, presumably as a far cheaper retrofit than a transformer swap.

Mike