Landline not ringing/answerphone not picking up

My landline stopped ringing some time ago, and the answerphone no longer picks up inbound calls. The lack of ringing/answerphone pick-up was not an immediately noticeable problem, as I do not receive many calls or messages at home.

Inbound calls can be taken if I see the screen on the DECT phone light up in time, and I can make outbound calls, though the line seems noisy/crackly. I have tried a different phone, which also did not ring when I called my landline number.

When I eventually noticed the problem and contacted VM support by phone, a line test was done, and I was assured that the line was working, which leads me to assume that the line ringing is something that cannot be tested reliably remotely.

I live in an area with patchy mobile coverage, so having a working landline is important to me.
Anybody have any suggestions as to how to investigate this further, or a solution?