Only usb-to-bluetooth dongles are likely to have the signal strength to penetrate walls. Mobile devices, and in particular earpieces and similar small devices, are likely to use weaker signals that rely heavily on being reflected off surfaces to get around obstacles.

And one such obstacle is the human body. Thus putting you phone on one side, and the ear piece on the other side may results in poor reception. But standing near a surface, be it a wall, a car, or something else, in front or behind may allow better reception as the signal can bounce off said surface.