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The 0121 dialling code covers Birmingham and the surrounding area.
Number 0121 423 5640 is potentially valid.
Ofcom records show that numbers starting 0121423 are allocated to BT. Note that due to number portability if an individual number has moved to a different operator this information is not publicly available.
Ofcom's last recorded change for this number range was July 1994.
Phone numbers beginning 0121 are geographic numbers.