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The 020 dialling code covers London and the surrounding area.
Number 020 3435 5718 is potentially valid.
Ofcom records show that numbers starting 0203435 are allocated to Gamma Telecom Holdings Ltd. 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 May 2009.
Phone numbers beginning 020 are geographic numbers.