I have the same problem. No-one I call can hear me, and when they do, it's always very faint and muffled. I don't use a cover and I have cleaned the microphones so I don't think that is the issue. I did the test with the voice memo, where the voice was clear, but it still is massively unclear on calls., The first person I contacted from Apple Support said it was a problem with the carrier (completely untrue and incredibly unhelpful) as I have made the same call with the same sim switched into another phone right after the failed call (from the same spot) and there I can be heard perfectly clearly. I have had the phone for two months (brand new) and regardless of whether I call from within a building, road, park, no-one can hear me.
Regardless of all the other uses of the phone, it is completely useless as a phone if it can't be used to make and receive calls. I have also read a number of other threads going back to Oct 2017 where other Apple users complain of the same problem - mainly iphone 8+ and 7 users. I think there is a big problem with the mic in this particular phone and Apple have instructed support to fob it off with idiotic responses like "it's your carrier" and generally deflect from the problem. They even suggested I use headphones or the speaker to make a call, which is clearly NOT a solution. I have made a genius bar appt, let's see if they say anything constructive. I have begun to hate Apple and its products because with problems like this, they are simply not worth the exorbitant prices they charge. I think the quality has drastically fallen over the last few years and Apple is not what it was and this is for certain the last iPhone I will have. I've had an iphone since 2008 and until the 6 came out, never had any issues with it. Now it's the exact opposite. (The first 8+ I got broke in less than a week and they had to replace it).