OK, attempt this:
I had been using Polycoms (601, 501, 301) before I got my Grandstreams about 6 months ago (GXP2140 (2) and GXP1405 (4) and 1 GXP2200EXT (sidecar)
Anyways I still have my tftpboot on my local machines with the configs and looked through each phone for a clue -- Taint there (you can't do it for one phone only) so I went and looked at the sip_318.cfg (the last working version for the 601, 501, 301 phones -- they want you to buy their new stuff for more $$, which is why the switch to GS)
in the SIP_318.cfg I found this:
<sound_effects se.stutterOnVoiceMail="0" se.appLocalEnabled="1" >
So if you change the stutterOnVoicemail="1" to a "0" like I did, that should take care of your question, but beware this is GLOBAL to all your Polycoms thatuse the SIP_318.cfg information
Newer firmware and Server/SIP config files may allow for individuals to set the stutter, but I did not see it. You could try the line above in just one phone. It could take it and work, it could take it and ignore it as invalid per phone, or it could send your phone into an endless reboot loop as an invalid cfg file.
I provide what I have but make no guarantees it will work per phone but in the sip_318.cfg it will work globally for all.
Hope this helps and Good Luck
p.s. as far as I know there is no tone for missed calls. did you use this to mean a missed call that left a voicemail?