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1.
1- Dialing code used for the majority of the Birmingham area.

2- Rhyming slang used by one of my sad bastard telephone engineer mates to replace the phrase 'do one'
"I've finished work now, so i'm gonna 0121"

"This guy asked for a free phone extension, so I told him to 0121"
by Mattclarke83 July 19, 2005
 
2.
The area code of Birmingham, UK.
Everyone who lives in Birmingham has a phone number starting with 0121.
by Greg March 20, 2005
 
3.
0121.... Do one.
Often used by the chav population given when asked for ones phone number.
"Can i have your number?"
Sure!... Its "0121.... do one"
by ZachDawglol August 03, 2012