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
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.
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