Polycom offers a tool to do that with their Polycom Productivity Suite, which is an additional feature set available as an additional purchase. Call recording is currently only for the Polycom Soundpoint IP 650 and 670 phones, as these are the only models available with a USB port. Calls are recorded locally to a USB storage device, which can be an external hard drive or USB flash drive. Not all drives are supported, however, so be sure to consult Polycom's USB compatibility guide before purchasing.
Once you have the phone in hand, you'll need to install the license for the Productivity Suite, which requires a boot server. First you'll need to download the license file from Polycom's portal. Once you have that, you can either send it to our support team and boot off of our boot server or install it in the license directory of your own boot server. Additionally, you'll need to specifically enable the Call Recording feature (and any other Productivity Suite features that you're interested in using).
To use call recording once the license is installed, plug your USB storage device into your phone. If the USB device is supported, you will see how many hours of storage are available displayed on your phone. To make a recorded call, dial the number as usual. You'll see a "More" soft key, which leads into the Record menu. Press the "Start" soft key when you want to start and the "Stop" soft key when you want to finish the recording. There's a "Pause" soft key for pausing. To review the recording, you must connect your USB device to a computer, where you can review or transfer the .wav files to more permanent storage. There's unfortunately no way to listen to the recording on the phone, which would be a nice feature. In my testing, a 20 minute phone call took 39 MB, so plan your disk space according to your needs.
Getting the call recording feature enabled in the first place is a little complicated, but actually using the feature is pretty easy. This isn't a solution for recording every call, but it is an inexpensive solution for users that need to record the important ones.