When the mobile phone communicates with the cell tower, it modulates the RF carrier - its this modulation that you hear as the chirping noise. This generates two components - the RF at whatever MHz, and a low frequency magnetic component, which falls away quite rapidly away from the mobile phone. This low frequency magnetic field is simply the nett current flowing within the phone from the battery, and is only at audio frequencies. The problem that Mojo has is it's strength - the small size, which means the unit gets closer to the magnetic field. If the mobile phone has a small magnetic loop internally, there are no problems.
To be honest, we have spent two years chasing our tails assuming it was RF - but now I am reasonably certain I have been looking in the wrong area (it's not RF) then we can look at other possible solutions. I am posting my findings about magnetic screening so others can experiment or offer suggestions.