Apple iPhone In phone mode dial *3001#12345#* then press CALL. The Field Test Screen will appear. The signal bars will now display the signal in a dB reading. -51 is full signal, -105 is no signal. When testing phones in a 3G environment it is possible to determine the frequency being used by selecting: UMTS Cell environment, then select UMTS RR info. The screen displayed will list the Uplink and downlink Frequency in code form. Uplink frequency on the 850 MHz cellular band will list an Uplink Frequency code between 4132 and 4233 and a Downlink Frequency code between 4357 and 4458. If you are operating on the 1900 MHz spectrum the Uplink Frequency code will be between 9262 and 9538 with the Downlink Frequency code being between 9662 and 9938.