BTW, this script can be considered as a workaround solutions for caller-group ringtone as all it does is monitoring the calls. The race condition cannot be prevented in monitoring mode. Though I've not encountered the problem, but someone else did, and in fact it's possible.

I'd consider making a package out of it if I can figure out the to control the calling flow so as to prevent the race condition.

Please advise if you could find any hints. Thanks.