NEVER EVER allow a builder or electrician to supply or wire up anything to do with either POTS telephones or network cabling. In my limited experience I've seen so many total disasters that take far more cost and effort to unscramble that any additional costs to use properly qualified people in the first place.
A few years ago I moved into a new build estate house.
On moving in-day, we had problems with electrics/plumbing - no hot water. While we waited for the builder's man to come and fix it, the BT engineer turned up to figure out why the phone's weren't working either.
I forget what the hot-water problem was, all I remember is the builder's man telling me he wasn't sure the boiler's pilot light was lit as he "couldn't see it". I pointed out that was daft, the fact the radiators were hot proved the pilot was lit. The builder's man stood his ground however and finally, in desperation, having reallised his incredible and toal stupidity, I ordered him off the premises.
Then I noticed the BT engineer, with head in hands and staring at the floor with a pained expression. I thought he was having a bad headache, but he simply asked "has the electrician gone", I answered "yes", and the BT man exploded in hysterical laughter. He'd heard the while debate about the pilot light and had been barely able contain his laughter.
Needless to say, it took the BT man another hour or so to sort out our phone wiring which was an absolute mess - but I gather that was par for the course for that particular builder (... ... ...). It all paled into insignificance a year or so later when the roof had to be stripped and relaid, but that's another story for another forum.
edit: Upon reflection, remove the builders name, they('re) lawyers can be a bit sensitive - 7LM