FreePBX provides many features but out of the box some of these will not work without some changes to the default Cisco/Lisksys SPA phones.
1. Your Dial plan should allow you to dial * plus at least 2 digits.
a) Can dial things like *97 or *98 for voicemail access
b) *100 to transfer calls to voicemail or dial directly into someone's voicemail without calling them
eg. (*xx.|xx.)
2. Your dial plan should also allow ** plus atl east 2 digits - this is used for Directed call pickup. Dialing **100 from a different extension will answer a ringing phone at extension 100.
eg. (*xx.|**xx.|xx.)
3. For Paging and Intercom to work you need to set "Auto Answer Page" to yes
4. Delete any vertical service codes that conflict with your assigned FreePBX feature codes