i tried that with some sugru on an old phone (samsung s10e) and it does a really good job of blocking the microphone.

if you have a case on your phone its a lot less destructive too since you can just stuff the sugru into the microphone hole in the case. the case i was using was soft rubber so it was easy enough to pop out the corner of the case to be able to use the microphone for a call.

that wasnt my daily phone at the time though so im not sure how well it would work in reality. i could see myself forgetting to pop out the case when i get a call and the other person just handing up before i realised what was going on.

it also doesnt work on every phone. i tried the same thing on a pixel 5 but blocking the mic hole did nothing, but that phone uses an under screen speaker so maybe there is something similar going on with the mic