It sounds reasonable that you could have a database problem. Twice I have had Ip phones go offline on an XTS. The display was still there but when you called the phones you get nothing, no fast busy, no ringtones, and the digital phone that called the ip phone would lockup. The only cure was to power down the system and turn it back on. Oh and a config ip card slot dump showed the phones in service as well.

www.myrandomviews
"Old phone guys never die, they just get locked in some closet with an old phone system and forgotten about"

Retired, taking photographs and hoping to fly one of my many kites.