Oh dear, looks like the same issue has happened again. Been working fine since I got the repaired Cosmo back, but yesterday noticed I couldn't hear calls except on speakerphone.
Not trying anything more than reboots at present, if I have to go as far as factory resets I can see me switching to another device permanently. As much as I like the keyboard (despite it's quirks), the rest of the device does let the side down a bit.