Do you want to be able to do something like
   echo "how are you" | mail 1@voicemail.alphanet.ch
and hear the synthetized voicemail message to voicemail account 1 ?

1. sendmail configuration (change voicemail.alphanet.ch to the
preferred domain)

/etc/sendmail.cf:
# voicemail (would need to run under non root UID)
Mmvm, P=/usr/lib/mgetty/send_mvm.sh,
      F=DFMhul9S, S=52/31, R=20/40, M=1000,
      T=DNS/RFC822/X-Unix,
      A=send_mvm.sh $u

/etc/mailertable:

# Update .db file with this file and:
# sendmail/makemap/./a.out hash /etc/mailertable < /etc/mailertable
# [ ... ]
voicemail.alphanet.ch       mvm:voicemail

2. special script

see send_mvm.sh in that directory

BUGS
   - sendmail 8.8.7 kills messages bigger than M setting (1000)
     without notification.
   - might need hand configuration, look in the file.
   - this needs to run under mvm's UID, currently root.

