DX followed by a short number and a name of place

Seeking truth


I am reading a letter from a law firm.

On the upper right side of the first page of the letter, there lists the name of the firm, the address, the telephone no. , the fax no. this "DX: 42xxx Cheapside", and the website.

I wonder what this "DX" might mean. But I have no clue. The simpliest guess is the word "distance". But what does it has to do with the short number following.

Ps: According to the on-line dictionary, "Cheapside" may mean a name of a street.

So, I need your help.

Thank you in advance.
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