Samsung S7 speaker phone not working + echo in call Phone: Samsung S7 SM-G930F
Problems:
1) Person I am calling or that is calling me hears delayed playback of their own voice
2) When speakerphone is activated a very loud noise is heard from the other caller's speaker making it impossible for them to hear me speaking

When I'm in a call (cellular service) and I activate speaker phone the other person can't hear me (I can hear the other person perfectly fine when on speaker phone). When I call (via regular cellular service) the people I call often report 'hearing an echo' in other words 'hearing themselves talk' , I have already changed providers which did not help. The 'echo' is always there from beginning till the end of the call from what I've heard.

I have a Samsung s7 SM-G930F. Previously had android 8.0 installed (which was the OS that came with the phone originally. I recently installed a different ROM (Lineage OS 16) but this had no effect on the speaker phone problems.

I tried starting the phone in safe mode as suggested in this video: https://www.youtube.com/watch?v=Z6qcYwyTl-w
But this didn't have any effect on the problem (same 'echo' still there). Suggesting it might be a hardware problem (assuming what's being said in the video is correct)

The following post suggests another possible cause:
"Apparently there is some kind of rubber band around the microphone that keeps the sound in the same place" (source: https://us.community.samsung.com/t5/Galaxy-S-Phones/S7-Other-side-hear-echo-on-phone-call/m-p/119530#M7152) . I think this might also explain the 2nd problem (loud noise on other caller's side when speaker phone is activated). Does this seam probable to you? 

Voice recordings work fine. The 'echo' doesn't appear to be present for all people, I'm told it has approx 1.5 seconds delay (the other caller hears their voice back with approx 1.5seconds delay).

When I start a call in whatsapp and talk to the other person they say they have no echo and can hear me just fine, if I activate speaker phone I can hear them properly but they say they have the echo again and there is a lot of background noise coming from their phone (sounds like 'some sort of electrical interference'), the 'echo' gets played back louder on their phone when I activate speaker phone, so loud that they cannot hear me talk.
When I turn speaker phone off on whatsapp and continue talking the other caller says the echo persists for some time (maybe 8 seconds) after which they hear a 'tick' sound and the echo disappears (also the 'electrical interference' background noise disappears immediately as soon as I turn the speaker phone off).

It happens regardless of how many 'bars' of connection I have.

Could it be a broken microphone? Could it be an issue with my antenna/connection?
What tests could I perform to track down the cause of the issue.

i have tried with and without the case (I thought the case might have a negative effect since the s7's speaker and mic are right next to each other)

Would love to hear your input
Thank you very much for any suggestions!
I got exactly the same problem with my Phone(Oneplus 5). 
I've tried a lot of things software side but nothing helped.
Due to my work, my phone was exposed to flour and I got flour in every opening of my phone, therefore also my charging plug didn't work perfectly. So I replaced the charging plug and with it the bottom microphone. Afterward all worked perfectly again.

So I guess your microphone is broken too.

Hope my answer helps.