I'm pretty confident they will find his ring trip as it is a fault that is as old as the telephone itself and can be detected even on an old fashioned analogue meter.
Moving to DV on a line with poor insulation is just masking the fault, for a while. Moving to DV on a line that tests okay isn't. These are different faults.
Depends. If the fault only arised recently then they may be able to find the fault. But if the fault was always there from the very beginning then there is little hope in finding the solution.
For example, this famous problem always existed in Dial-Up by design as back then we had modems. My first ADSL 1Mbps service was with Tiscali in 2003 and here where I live in Central London last 32 years this problem always existed! Never found a solution. The solution for me was to simply disconnect the analogue phone and not bother to have it plugged with the master socket.
It persisted till the very end. I knew that my general broadband was usually stable but was mainly with the phone as I remember that even under ADSL when I had my SNR margin capped to 9dB that completely fixed the broadband drop outs except that the SNR did drop early mornings to 2-3dB before recovering back. This held my connection but solved the drop outs.
But the phone causing drop outs was another separate problem for me. After FTTC my general broadband drops were completely resolved as now my connection is maintained even with a 3dB SNR. But phone calls, pickups or connecting them to filter/faceplate always triggered a disconnection similarly to the behaviour experienced in Dial-Up.
A Sky engineer came and he couldn't find a fault and an Openreach engineer also couldn't find a fault. Anyway, by then we already ditched the landline and used only mobile.
Only after the Digital Voice migration with BT it resolved the problem naturally. Yes, it is masking the fault. But this will be the case for loads of customers after 2027. Many people will be secretly having the problem but they won't realise it because they simply won't know they have it as DV will not trigger these issues! That is my suggestion to the OP in the scenario that Openreach fail to fix his problem..