Hi,
what type of voicemail do you have? A Coral card eg iCMC, uCMC, WiCMC, Simplcity, or a 3rd party voicemail?
Regardless you can always create public libraries to do this.
Lib Num = 6000 (for example)
Dial Num = xxxx
Out Tk = yyyy
Where xxxx is the xtn number of person you want to leave the msg for and yyyy is the pilot number of the voicemail (this could be a hunt group for 3rd party or Simplcity, or the APDL port for iCMC,uCMC,WiCMC). Depending on the voicemail you may need to alter the Dial Num if the VM is expecting digits before/after the xtn number.
Using iCMC/WiCMC/uCMC and 12xS/28xS phones with softkeys you may get an option on a softkey but I'm not sure so you'll need to check. Obviously SLS ports don't have this option so would still need to use Pub Lib's.