We broke the task of testing cordless phones into 5 parts: Range, Sound Quality, Ease of Use, Battery Life, and Phone Features. Some of these categories were much more subjective than others, but our goal was to have a rigorous, objective test for each attribute that we scored.
Our first set of tests focused on range. We bought the longest phone cord that I have ever seen and ran it from inside the house to a folding table set up on the street. We called a Google voice account on a laptop from each cordless phone and had our tester start walking until his voice became unintelligible. We marked out distance with a surveyor's wheel and started walking. And walking. And walking.
In all seriousness, the cordless phones we tested use the Digital Enhanced Cordless Telecommunications, or DECT standard. This means that the line of sight range of these phones is really long, in excess of 700 ft. We ended up getting a bicycle to make life on our testers a little easier, and measured out the distances at the end, with them marking out where the phones cut out. All the phones had a much higher range than we initially thought, and we ended up having to walk up the hill at the end of the street before they even started to cut out. We ended up cutting off the test at 840 ft, running out of room to maintain a direct line of sight of the base.
We felt that, while interesting, this was not the most useful test for the majority of people using cordless phones. Heading back to the drawing board, our next test was designed to make it a little harder for the phones, and more applicable to the real world. We wanted to add some walls between the handset and the base of each phone. Using the surveyor's wheel again, we measured out and placed a marker every 40 ft. Next, we called Google Voice and sent the call to voicemail, and started walking with the handset out of the house and down the street. Our tester played an audiobook (The Tibetan Book of the Dead) into each handset as he walked, calling out the number of each marker as he passed. We listened to each voicemail until the recording was not longer audible, and determined the effective range. As we expected, the range was severely reduced when the base was separated from the handset by several walls
To test the sound quality of each phone, we left a message on Google Voice with each handset and had a panel of listeners rate the quality of each message. They rated the quality blindly, without knowing the make and model of each phone. The message was a passage read from a book, that our speaker had practiced multiple times to ensure consistency. We repeated this test to make sure that the results converged on the same scores for each phone.
We split ease of use into 2 groups: Basic and advanced functions. We tried to use each phone in a dark room, measured how far away we could read the text on the screen, and measured the ringer volume at its maximum with our SPL meter. We also looked at how difficult it was to place each handset on mute and put a call on hold.
To evaluate the different features of each phone, we went through and made a list of all the features that the combined group of phones had. We then assigned full marks if a model had that feature, and summed the cumulative scores to pick a winner.
We wanted to test how the battery on each phone performed and set up a talk-time test to find out. Some phones have an "ECO" mode that preserves battery life when close to the base, but we wanted to see the worst case scenario for each model. We set up a handset from each model of phone around a speaker and played music until each phone died. We also made a note of when each model signaled that its battery was low.
For more information, and to see how all the phones compared to each other, check out our detailed Cordless Phone Review.