This one has been perplexing me for a few days now. I will also preface this by saying I inherited this system and am trying to learn as much telephony as I can over these past few days.

I set up one of our phone sets to forward calls to an external number. If I call the set's number with another internal phone line, it forwards to my cell phone, no issue. But if I try to call the set's number from another external number (like another cell phone), it doesn't even ring on the caller's end and doesn't get forwarded. The only indication that anyone called is that it shows up in the call logs on the phone set. Just to add to that, if I cancel the forward while trying to call the phone set, it will begin to ring as normal. Redirection is enabled on the set.

Any ideas?