"They mostly ring once at night... mostly."

[#1]

Very surreal query coming up.
If someone rings my home line, the phones in the house only ring once when an incoming call connects, and then go silent. Call quality is fine if we pick up though.
I don't think it's the the handset because we have two handsets, one plugged directly into the BT master socket and the mains, the other's a cordless phone upstairs which is talking to a base station. They both do the exact same thing. Ring once and then stop, as if someone's already answered. I've checked both phones and they're not set to do this, or set to auto-answer after one ring.
The phones are still ringing according to the caller but the handsets ring once and then don't ring again. I can take the phones to another house and test them there but is there any possibility this could be a line issue?

[#2]

Hi Tim,
It could be a line issue.  Make sure that you have tried using a different filter directly into the test socket and trying the corded phone at a different house would be a good move.
If that doesn't help, get in touch with us and we will raise it with Openreach.

[#3]

I've changed the microfilters with new ones and it hasn't made any difference.
It's really not annoying me enough to raise as a fault (I'm working away a lot and most people who need to ring me ring the mobile anyway).
In the long long term, I'd much prefer to be using use SIP than faff with legacy telephone wiring in a house that I don't own. If it weren't for the fact ADSL needs a traditional phone line I wouldn't even bother having a phone line.