Changing a cellphone's frequency to 900 MHz could potentially cause interference with a 900 MHz cordless phone, similar to the echoes heard in older models before frequency hopping technology was introduced. Most modern cordless phones operate digitally, which might result in static rather than clear interference. Frequency hopping is a technique used primarily in 2.4 GHz and 5.8 GHz phones, likely absent in many 900 MHz models. While GSM technology scrambles signals, making interference less likely, the 900 MHz frequency is commonly used for cell phones outside North America. Overall, significant interference is unlikely, but the exact effects depend on the technology used in both devices.