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A hum is heard on calls on a particular phone

Core Issue

The hum sound can be due to a number of factors, which include:

  • a defective power supply

  • electrical interference

  • poor cable connectivity

  • digital errors


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.

Power specifications:

  • 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

Refer to Regulatory Compliance and Safety Information for the Cisco IP Phone 7900 Series for more information.

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.