Look at the RCO prompt in the set programming. You have 3 systemwide settings for # of rings, it sounds like one of them is set for 1 ring and assigned to that phone. Print out your RDR data from LD 15 (print in LD 21). Check the values for CFN0/1/2 and DFN0/1/2 -- they equate to RCO 0/1/2 on the set. Be careful about changing the value in RDR, though, as it affects every phone with that RCO; you might change the phone to another RCO instead.