I’ve been having very similar problems with my Murena FP5 since I purchased it about six months ago. Telephone, Signal, Element and Element X calls on speakerphone are reported to be very poor quality by the other people. For Signal and the Elements the audio reception frequently cuts out with brief periods of robotic sounds. When using the earpiece or USB headphones the received quality is better, but still with frequent cut outs and robotic sounds. This is not a problem with my WiFi or Internet access.
However, telephone calls via the earpiece are fine, as are voice notes on Signal and the Elements, via speakerphone or headset, as well as audio recordings via apps. Playback of voice notes and media in general is perfect.
The headset works fine on my laptop.