There are several ways that Radio Frequency energy radiating through space can interfere with other electronic devices. The most obvious one is if the other electronic device is a radio receiver tuned to the same frequency, and yet this is the most rare explanation. The most common form of interference occurs when the RF energy is turned into conducted voltage and current on a wire that is inadverdantly acting as an antenna. This RF power flowing on the wire is still at 800 MHz, but when it flows along the wire and hits the semiconductors in your electronics it gets converted to low frequencies through various processes including AM demodulation and through mixing with other oscillators inside the equipment. You see, most ICs, all diodes and all bipolar transistors contain PN junctions. These PN junctions are simple AM demodulators and when you put a high frequency EMF across them, they rectify the RF which converts much of the power of the RF to a baseband (low frequency) signal and this appears in your equipment. The stronger the signal the more you get baseband voltage appearing in your device.
Many cellphones use the GSM communications standard for voice. This is a Time Domain Multiplexed system so your cellphone is transmitting with very rapid on/off pulses, it is not transmitting steadily. These pulses are being picked up by nearby wiring in the monitor and other equipment, they are being rectified and so you have electrical energy at the pulse frequency which is in the audio range, I think around 220 Hz. So if this happens to audio equipment you hear buzzing when your cellphone is transmitting.
As most Ham Radio enthusiasts know, when you get interference of this sort, it is a sign that the device that is receiving the interference was not robustly designed. All governments issue radio licences to those controlling the radio equipment (in this case the cell system operator) which grants immunity to complaints of rectification interference. This immunity is not, however, extended to license-free radio devices (such as those certified or verified as Intentional Radiators under FCC part 15 in the USA) which includes most of the radio transceiver projects discussed on these boards.