The power supply spec is:
QuotePower Supply Type Universal Switching with Fixed US, EU, UK Style Plug Prongs (Model Dependent)
Input Power AC Input: 100 to 240 Volts 0.3A 50-60Hz (26-34 VA)
Output Power DC: +12V 1.0 Amp Max
I have two OBi110's here in the UK where the power frequency is 50Hz and I have no hum.
Having said that, it does sound like a power or noise induction problem. Your brother might try a different power socket or a different power supply, if he has a compatible one. Also, noise can be induced into to telephone wiring if it runs near to power wiring.
Do a search on this forum for "noise", there's been quite a few posts about it.