get_esn
Description:
Gets the Electronic Serial Number, as reported by this ModemCdma.
The ESN is superceded by MEID, but still used in older devices.
Warning: 
      
        
    
    The returned value is only valid until the property changes so it is only safe to use this function on the thread where this was constructed. Use dup_esn if on another thread.
Parameters:
| this | 
           A ModemCdma.  | 
      
Returns:
| 
           The ESN, or   |