The following wiki-page http://www.zabbix.com/wiki/howto/config/alerts/notificationsusingasterisk describes basic integration of zabbix to asterisk.

I´m not capable to add ideas this wiki-page because it seems that this is only possible for zabbix-developers.

The following features would be a nice:

• create a voice-message using a speech synthesizer like festival : http://www.cstr.ed.ac.uk/projects/festival/
• provide a possibility to add a phone number to user´s media-profile
• make a call to a configured user
• provide the possibility to acknowledge the alert in zabbix by using a dialmenu
  ("...hit 2 to acknowledge the alert, hit 3 to restart service,...")

Regards
Marc