Strange things occurring?

Am in UK so aimed at BT Engineers, others of course ae welcome. I knew I should have kept my test equipment when I retired back in 1992. Those were the days when you got a fault repaired in one day two if it needed a new cable service pipe.
Freestyle 650 twin cordless instrument {BT jargon for telephone contracted to BT landline.
Using extension fitted by BT. No line on both phones.
My son has broadband internet (not BT’s) that still connects.
Problem: Lost dial tone on phone. Lost two calls during call time now no dial tone at all.
Tests: Changed internet filter: still no dial tone
Switch instrument off for a time, turned back on of course. Still no dial tone.
Took instrument out of the extension socket fitted to first line socket coming in to property connected to main via extension lead to power main unit. Still no dial tone.
Disconnected broad band, connected instrument direct to first socket. Still no dial tone.
Finally fitted to the first line socket a corded phone kept for such times and still no dial tone and I know that corded phone works. Also checked the socket the instrument was originally in. Still no dial tone.
Any suggestions as to why we still get internet but not the land line telephone side of it.
My son says data is getting through hence the internet still works.
Ex ST G.P.O. /BT direct labour group London Western District who now knows he should have kept his testing equipment