UC520 can't conference more than 3 callers.

(1) These commands must be added in config mode
sccp local Vlan10
sccp ccm 172.16.1.1 identifier 1 version 6.0
sccp
!
sccp ccm group 1
bind interface Vlan10
associate ccm 1 priority 1
associate profile 1 register ConfDSP
!
dspfarm profile 1 conference
codec g711ulaw
maximum sessions 1
associate application SCCP

(2)Then add these commands to telephony-service
!
telephony-service
sdspfarm units 1
sdspfarm tag 1 ConfDSP
conference hardware

(3) last create the ephone-dns/ I also added a meet-me number

ephone-dn 6 octo-line
number AAAA
conference ad-hoc
no huntstop
!
!
ephone-dn 7 octo-line
number 3310
conference meetme
no huntstop