Persistent line interference, occasional loss of services

Hello - hoping that the clever folks here can diagnose a recent and ongoing issue with my landline phone and internet at home? Apologies if this is not the most appropriate section in which to post.

For the past week, a horrible crackle has appeared on the phone line, varying in degree but mostly making the phone unusable for calls. Anyone who calls me can also hear this immediately. On a few occasions I've been unable to get a dialling tone at all, with a "check phone line" message on the handset, and friends have called my mobile to tell me that they hear the "unavailable" message.

Less frequently, but still much more often than is usual, my internet service (FTTC) has either slowed terribly, or dropped out for anything from a few seconds, to almost eight hours on the first morning that it happened.

I briefly spoke to a TalkTalk agent via their Live Chat service last week, who had me disconnect the phone and internet connections from the master socket in my home. He suggested that some kind of fault had been identified but that an engineer visit would be required to identify the issue precisely. I didn't arrange this at the time with the intention of making sure none of my own equipment was at fault first.

After removing the faceplate from the master socket, I've now tested both the existing phone, and an older base unit with wireless handset, connected to the test socket alone. The interference is still there throughout.

Does any of this suggest something obvious at fault? Is there anything more I can test before arranging for an engineer visit?

Grateful for all advice,
Sarah

Unplug all broadband kit from the phone line and power. Then connect a corded phone to the test socket,reason for a corded phone is to rule out interference on the wireless part of the cordless phone or interference from its power supply.

Thanks, Andrew - I think that there might be an old corded phone in the loft, I'll go digging tomorrow and give it a go.