Possible Clues:

• Letters near 0
• Abbr. on a phone's "0" button
• Abbr. near a 0
• Abbr. on some dials
• Phone abbr
• Abbr. above '0'
• 0 on a phone: Abbr.
• Phone abbr. near 0
• Letters under TUV, on many phones
• Letters next to 0