Auto population of Subject line on a phone call

So I'm attempting something very simple - All I want is to populate the subject line (title field) when a specific value is chosen on a custom field drop-down.

something like:

drop-down list
A
B
C
D

Subject line:

W
X
Y
Z

So when A is chosen from the drop-down, W is entered into the subject line, and so forth. Pretty simple. I am using the Field Changed Function on the phone call form for testing, and will eventually make this global once the kinks are worked out.

Code is:

function fieldChangedAddSubject(name)  {    if (name == 'custevent_call_type')      var callType = nlapiGetFieldText('custevent_call_type');      if(callType == 'A')   {    nlapiSetFieldValue('title', 'W');   }   else if (callType == 'B')   {    nlapiSetFieldValue('title', 'X');   }  etc...  }