Hi @abi73 I'm sorry the first engineer didn't get it this fixed, unfortunately, intermittent noisy line faults can be tricky to repair, if it's still crackling in the test socket then you can be sure its something on the outside network.
Thanks to everyone who rightfully said phone crackling etc was an external issue. Engineer today said 50 year old wiring from pole and old junction box should be renewed automatically and didn't understand why it wasn't done by engineer 1. All renewed, new internal connection, problem solved. Just waiting to see if I get charged for the first failed job.
Hope my experience helps others.