From an extension, in the Call Forwarding and Voicemail | Work Hours rules, I am attempting to forward a call to multiple external "Numbers" (not "Coworkers"). I have it set to "Ring all at once" and have 5 numbers listed. These numbers are cell phones.

The recipients report that only two of them are getting the calls forwarded to them. 3 are not getting calls at all. Is there a limit set to external numbers to receive forwarded calls from a single extension? If so, what is that limit. Is this documented anywhere?