These instructions were written for Polycom SoundPoint IP 331, SoundPoint IP 650, and VVX 410 phones connected to FreePBX 10.13.66, but they will probably work with other Polycom phones and versions of FreePBX.

Use Basefile Edit in Endpoint Manager to edit the MAC-Features.cfg file. Add the option up.oneTouchVoiceMail="1". My resulting up section looks like this:

<up
   up.25mm="1"
   up.25mmRealTime="1"
   up.accessibilityFeatures="0"
   up.analogHeadsetOption="0"
   up.audioMode="0"
   up.callTypeOrderVoice="0"
   up.cfgWarningsEnabled="1"
   up.displayOperMode="0"
   up.handsfreeMode="1"
   up.headsetMode="0"
   up.hideDateTimeWhenNotSet="0"
   up.idleStateView="0"
   up.localClockEnabled="1"
   up.manualProtocolRouting="1"
   up.mwiVisible="0"
   up.onHookDialingEnabled="1"
   up.pstnSetup="0"
   up.simplifiedSipCallInfo="1"
   up.useDirectoryNames="1"
   up.warningLevel="0"
   up.welcomeSoundEnabled="1"
   up.welcomeSoundOnWarmBootEnabled="0"
   up.oneTouchVoiceMail="1"
>

Rebuild the template and reboot the phone. Pushing the message button now dials the voicemail directly without displaying the message center.