Most accidental cellphone detection comes from non-linearities in the circuit. Although the voltages that you can pick up are quite large, they are very high frequencies (900 or 1800 MHz in Europe, 1900 MHz in the US) so very small capacitances will short them out.
You should start with a high speed diode, or with a transistor that has a transition frequency (ft in the datasheets) that is at least 1 GHz if you want to get any gain at all at GSM frequencies.
The buzz that you get on the speakers is because the GSM phones only transmit in 58 μs bursts, 216 times a second. The amplifiers behave differently during the burst, and so the repeat rate of the burst is clearly audible.