The hum sound can be due to a number of factors, which include:
a defective power supply
poor cable connectivity
In order to resolve the hum issue, move the phone to another location away from the electrical interference. If you still hear a hum sound, swap out the phone power supply. The electrical interference can include interference from a mobile phone.
100-240 VAC, 50-60 Hz, 0.5 A with an AC adapter
48 VDC, 0.2 A with the in-line power over the network cable
In order to ensure the best performance and best immunity to the potential of an AC hum, Cisco strongly recommends the use of either Cisco in-line power or the CP-PWR-CUBE-2. The -2 power cube is designed for the CP-7970G color phone with the intention it can also be used on all other Cisco IP Phones. As it turns out, it has additional filtering which eliminates some hum on Plantronics headsets at several locations.