I have ring group and when all SIP clients in this group are busy call go to Destination if no answer. I know it is normal. Is it possible to ring continuously in this case? (Now i use sip client, which is always online, but this is not stable and good solution)
You could use a Queue with the Ringing option to play back ringing to the callers.
Ring groups are not a native Asterisk feature. They are implemented by FreePBX in terms of either queues or a sequence of Dial application calls. Please use http://community.freepbx.org/ for peer support with the use of FreePBX features.