Certain older iPhone models, including the iPhone X and iPhone XS, turn the "Hey Siri" functionality off when audio is playing and your device volume is set higher than ~25%.

This is an iOS limitation that unfortunately we are unable to fix ourselves. The only workaround is to: (i) turn your device volume down lower than 25%, or (ii) temporarily pause the audio when you use "Hey Siri" and Portal at the same time on these older devices.