I just heard (2/7/2109) from my audiologist, who says that Phonak will have a firmware fix to be released the week of February 25, 2019 that is supposed to fix the communication issue with iPhone XS. The release is in a software program audiologists use, called Target that allows them to update firmware in Phonak devices.
Check with your audiologist and make an appointment to have the firmware updated. Hopefully all of us on the "bleeding edge" of Phonak's technology will get the performance we were promised when we purchased.