How would you say 4721 if it were in something like an address or telephone number?

Posted by NegligentBat@reddit | AskABrit | View on Reddit | 253 comments

I'm currently in "An Inspector Calls" a play set in 1912 and I'm using an RP accent for the most part. But at one point I give a telephone exchange number of 4721.

I've been saying "forty seven twenty one" but I realized this might be an American thing and am wondering if I should say it "four seven two one."

Thanks for your help.