The round button with white circle is the call button while on the right is the volume control. On the other side is the power switch and to turn it on, simply slide it down to show green; red is power off. The top is a microUSB port for charging the headset internal battery. The Indicator light is below the earpiece, and if it shows three red flashes during powering on, it means that the headset's battery is low; repeated red and white flashes mean the headset is in pairing mode; while a solid red means that the M155 is fully-charged.
Before you power the headset, wear it first on the side you're more comfortable with. If the earpiece doesn't wear nicely, you may want to try the other sizes provided in the box. Power on the headset by sliding the power switch and you should hear a voice that reads out the available talk time for use. The pairing of the Plantronics M155 to your phone is very straightforward – simply turn on your phone's Bluetooth connectivity and wait for it to automatically scan for the headset, and select the Plantronics for pairing. Depending on your phone, you may need to manually scan for the headset instead.
One neat feature of the Plantronics is the hands-free answering or cancelling of calls. When a call comes, the headset will prompt you whether you want to answer or ignore. Just by saying out the word "Answer" or "Ignore", the headset will carry out your command accordingly. However, note that the voice commands don't always work, especially if you're in a very noisy environment or the headset simply didn't catch your pronunciation correctly. Alternatively, you can also answer the call by a single tap on the call button, and ignore the call by press and holding the call button for one second. The call button also offers other functions including redial (tap twice), voice dialing (two-second press and hold), reconnect lost Bluetooth connection (single tap if headset can't connect automatically), and activation of pairing mode (press and hold till red/white LED flashes).
The Plantronics Marque M155 has a operating range of up to 10 metres. If you leave your phone on the table and walked more than 10metres away, the headset will tell you that connection is lost. But once you are in range, the headset will pair itself automatically and will prompt you that it is now connected to your phone. In terms of audio quality, we can hear the other party very clearly on the other line, although at maximum volume of the headset, it may not sound as loud as you thought it would be. Well, there's one way to make it louder; just increase the phone's call volume.