Why Do Wireless Routers Interfere With Phone Lines?

By Dan Stone

Wi-Fi and cordless phones can interfere with each other when they operate on the same band.

Wireless routers and cordless landline-based phones can compete for the same radio bandwidth as each other, which can cause interference for both devices. In rare cases where the physical phone line is improperly shielded or damaged, Wi-Fi transmissions can interfere with the phone line. However, it is more likely that the wireless router is interrupting the radio communication between the phone unit and the transceiver base that connects to the phone line itself.

Radio-Based Devices

Cordless phones and Wi-Fi devices operate with radio technology similarly to how AM and FM broadcast radio work. Cordless phones and Wi-Fi devices are capable of both sending and receiving information: they are both transceivers. Radio devices transmit over a frequency range. The more signals that operate near or in the same frequency range, the more difficult it is for the receiving device to pick up a clear signal. In a situation where there are a small amount of regulated devices sending information in a frequency range (for example, broadcast radio), the transmissions can be spaced out far enough so they don't interfere with each other.

What Causes Interference

Receiving information isn't a problem, but devices that are sending information have to compete amongst each other for broadcast space. Interference occurs when multiple devices are broadcasting within the same frequency range. The receiving device will only get part of the data from the transmitter because the non-intended transmitting device is blocking out the signal. Digital devices can request that the origin device resend information, which slows down the data transfer, but even digital cordless phones need to operate in real-time, so if the information doesn't get to the phone receiver in time the conversation will be filled with static or cut out completely.

Only Shared Bands

A wireless router will only interfere with cordless phones that operate in the 2.4-GHz and 5.8-GHz bands. According to telephone hardware sales site 101phones.com, cordless phones can operate on the 900-MHz, 1.9-GHz, 2.4-GHz and 5.8-GHz frequency ranges. Wi-Fi routers can broadcast on the 2.4-GHz and 5.8-GHz bands: some devices broadcast on both. A Wi-Fi router will not interfere with cordless phones that operate on the 900-MHz and 1.9-GHz bands, but it can compete with phones that operate in the 2.4-GHz and 5.8-GHz frequency ranges.

Dealing with Interference

The easiest way to work around Wi-Fi router interference is to use a cordless phone that does not broadcast in the same frequency range as the router. However, replacing the phone may not be a cost-effective option, so you can adjust the router's settings to work in a different sub-channel. Most routers offer some sort of sub-channel control in the device configuration. The process varies between devices, but changing the settings may stop device interference.