The living room in my house is monitored by CCTV with audio. Will that be admissible in court?
The law 'doesn't care' how the call is recorded. What matters is whether or not you should inform / should have informed the participant(s) in the circumstances.
In circumstances where you are acting as an ordinary member of the public, in the course of a purely personal or household activity, not in a journalistic capacity, regulated business or other circumstances where the rules may differ:
In the UK it is not unlawful for a private person to record a phone call without the permission or foreknowledge of the other participant(s) - provided the recording is for 'personal use'.
If you intend to share the content of the call with a third-party or make it public, then you must inform the person ahead of recording it.
If you try to use a covertly recorded call as evidence in court, the court may or may not exclude it depending on the circumstances.