Bluetooth radios operate on the unlicensed 2.4 GHz ISM (Industrial, Scientific, and Medical) frequency band. Thus some appliances such as cordless phones, wireless networking equipment (Wi-Fi) or microwaves may cause some interference, especially if the distance between the headset and the phone is large.
Interference can be minimized by avoiding obstacles between the Bluetooth antenna of your headset and the antenna of your source device (phone, MP3 player etc.)
However Bluetooth has been designed to work in “high interference” environments and is much more robust than many other wireless technologies. Devices with Bluetooth Version 1.2, or higher, use a technology called “adaptive frequency hopping” to automatically hop from frequencies with a lot of interference to “quieter” frequencies. Bluetooth radios switch frequencies at such a rapid pace (1,600 times per second), and the data packets are so small, that interference from other RF sources is highly unlikely.