I'm also experiencing this issue on an 8 Plus and carrying on conversations has become beyond frustrating. I've had this phone for 6-7 months, and experienced the problem from day one. I'm stunned to have found this post and discover that not only am I far from alone, but to see threads like this have lasted more than a year with no resolution from Apple which is simply unacceptable.
I purchased my 8 plus fully unlocked at an Apple Store just days after the XS was released. I chose the 8 Plus mainly because I have issues closing the windows on the newer models that lack a home button. This may be because of my own physical disability, but regardless of the reason, I was satisfied with my choice until I started making calls. Nearly everyone I talk to was telling me, "You sound muffled, like you're in a tunnel, underwater, etc."
As soon as I switch to speaker, I'm told everything is clear, but this is not a solution as some calls need to remain private. Just 15 days after my purchase, I went to an Apple Store and everything tested fine. Calls to Apple Support as well as that visit to Apple both had me believing it was a carrier issue. After 3-4 weeks of frustration with calls, I finally switched carriers from Tracfone to Verizon.
I have better service overall but the problem remains. I went back to the Apple store several weeks later, and the new possibility suggested was to have them replace the antenna. We knew it wasn't a carrier issue since I switched carriers specifically for this reason. I left the store undecided about what to do, because I'd rather not possibly end up with a refurbished phone just after 2 months and $800+ (256 GB model) to get a new one. After discussing this with Verizon, they confirmed it is not an antenna issue as my Data speeds would be affected as well. I have excellent data speeds and not a signal internet or MMS texting issue.
Over the past 3-4 months, I haven't gone back to the Apple store, because many people that I talk to stopped mentioning the issue. I also make most of my important calls on a landline to avoid the issue, which is not a solution by any means. Obviously it’s sometimes not possible to wait or schedule a call for a time when I'm near a landline.
This week people have begun mentioning it again – I’m told the problem never went away, but they just didn't want to continue bugging me about it. I simply don't know what to do as I'm not a fan of Android OS and cannot trade the phone in for an XR or XS as I need the home button.
Purchasing a new 8 Plus is, as this point, the only way to ensure I’d actually be getting a new one, but that’s not acceptable to me since I’ve only had the phone for 6-7 months and this has been a problem right out of the box. Not to mention, this issue is so common that it may happen again with a new or refurbished model. I’ve tried all the suggested “fixes” – turned off noise cancelling, hearing aid capability, changed LTE setting to Data only, restored the phone, blown compressed air on speaker grills, - nothing changes until you use speakerphone for calls. Apple needs to address and resolve this issue ASAP!
Any suggestions for someone who is unable to just trade up to an XS would be greatly appreciated.