I have 10 extensions grouped into RingGroup with number "100" and "ringall" strategy. Only 4 of 10 extensions online. Someone call to 100 and 4 online extensions gets call and starts ringing. So, how can I get this call if one (or more) of 6 offline extensions gets online (until call is active)?
Probably you should use queue for such functionality, ring group is not really good for such case, but there is a hack to achieve what you need even with ring group.
First make sure that destination if no answer is same group. then make sure that "Ring Time" is configured for quite low value like 10 seconds.
In this case when call hits ring group it will ring only 4 available extensions for 10 seconds, after 10 seconds it will go back to same group and it will ring all available extensions at that moment, so if 1 additional extensions will go online, then it will call 5 extensions for 10 seconds and etc.