Top mic supposed to be work for noise-cancelling but there are some cases that it does not work properly. This is reson why there is noise or echo during call. This issue has reported as solved in 10.3.5.0 stable but I think some people are facng this issue in 10.3.6.0.
Please submit bug report with log via Feedback app so devs can look into it and hopefully this issue will solve in next stable update which will come this month.