You have a difficult loss for the Marvel phone function. One of the things that Phonak didn’t alert providers to from the beginning is that the audio that goes out to the other person is filtered by the hearing aid’s feedback manager. So if your feedback manager is cutting the high frequency gain dramatically to control feedback, the person on the other end isn’t going to hear you clearly. This is less of a problem for someone with more moderate high frequency hearing loss who is at a lesser risk of feedback, or for someone with more hearing loss in the low frequencies that allows them to tolerate a more occluding dome/tip (which again improves feedback issues).
You can potentially improve the experience of the person on the other end by getting custom tips made to help manage the feedback, and then setting the phone ear to whichever one has a better feedback measure. If this isn’t enough, you might consider moving to a different hearing aid which allows you to stream from your phone while using either the phone mic or a secondary device to send the audio back out. If you’re really loving the Marvel, that might be a sad move for you, but if you didn’t end up liking other current generation hearing aids with direct iphone streaming you could always take a step back to the Phonak Belong line and use an intermediary device.
Now that I’m typing and thinking and drinking coffee, you could also look into whether you could pick up a Roger Select and then route the call through the Select instead of through the hearing aid, allowing you to use the Select as the mic. This was a pre-Marvel option that I haven’t done yet with the Marvel since it just connects directly, so I don’t know whether some change in the Marvel broke that feature or not.