To me, it is hard to give out a good answer without knowing more about system. Assuming that they are CO lines, and not a PRI T1, first place to look is in Trunks, and see where the trunks ring to in the DIT setting.

Then find out if that number (Dialing Digits) is an extension number(DN), Phantom Extention (PhDN), or Multiple Call Group (MCG).

On another note, 15 seconds is LESS then 4 rings.