Hi, the https://wiki.debian.org/mtp already states a warning "Be aware that many smartphones will only enable MTP while the phone is not locked!*"
This is ambiguous and it took me a long time to figure out what lock this is really about and how to unlock it. It is not a screen lock or phone access lock. Typically, upon 'mtp-detect' when locked there will be a panic message that the mtp device cannot be connected. In Android (Android 9, Pie) one has first to activate 'Programmer Options' by repeatedly tapping 'compilation number' under 'About the Phone' under 'Settings'. These 'Programmer Options' become available under Settings / System / Advanced. There one can open 'Predefined USB Options' and allow USB to be used for file transfer. Then the mtp package will connect automatically and from Thunar the phone storage will be accessible!
