The easist way to set up voicemail access is to configure a call flow to 'Check Voicemail', as shown below:

You can add more than one internal extensions (extension number 555 is used for voicemail access in the above example) - or for external access, one or more external numbers / DDI's also - to access voicemail.  Please also ensure that the voicemail box is protected by, and has set, a pin access number.

If you wish to program a device to check voicemail insert the call flow number / extension into the appropriate device setting(s); it is not possible to list configuration for all SIP devices.