There are no such thing as telegraph poles really, but utility poles. Utility poles that carry only communications lines over ground are usually around 4M, except when they have to cross a road/driveway, navigable waterway, or railway, then maybe 5M . Add to that 2M underground. Poles that carry electrical power will be taller, and the more voltage the higher. I’d say they might be up to 20M tall, with 4M under ground, before they use pylons.