When connecting my iPhone to the Z-E3150 I can hear annoying audio dropouts

AW: Most problems originate from “aftermarket” cables not officially licensed by Apple. Therefore make sure, that either an original cable, or else at least a licensed Apple Lightning cable is used.

Otherwise, perhaps this little trick can help:
Power off the iPhone – Power off the ZENEC device.
Switch on the ZENEC device – switch on the iPhone and pair it. Unplug the Lightning cable, reconnect it – then everything should function normally.

Make sure, the iOS version of your Apple iPhone is always up to date.