Happy but Puzzled

My BT landline lost dialling tone on both DECT phone and corded phone and would not accept incoming calls. It was the same in the master socket (ADSL filtered faceplate) and in the test socket. Infinity FTTC was unaffected running on a filtered extension.
BT online help wanted me to connect my Homehub 5 to the test socket and connect to the internet. This I did, and as forecast phones became fully functional. I am happy that it worked but puzzled as to how it could work. Any suggestions appreciated.

Is your wiring still as in this post? Where are the phones plugged in?

Thanks for your interest. Everything restored to original connections as per the linked post. Only change is HH5 now replacing HH3 and modem. DECT phone to master socket, corded phone to filtered extension HH5 to filtered extension. All working OK

Very odd.

Something was putting a loop (short circuit) on the line. This will happen if you accidentally plug the DSL cable into one of the ethernet ports on your home hub.

Yet you say the FTTC was working as normal so it can't have been this.

Very odd.

Just read this post. I have been thinking about replacing my HH3+modem with a HH5. The reason would be to limit the number of wires and to improve wifi reception downstairs, as the line comes in on the first floor. Is it worth it? As a BT customer, is it worth seeing if they'll give me a discount on the HH5? Absolutely no problem with my current setup except the flaky wifi between floors.