I checked the address on Zillow, it’s a house that was sold in 2013 (no price given). Also has a weird tax history.
KMKhanLaw.com had the (202) 279-0806 number since at least 2013, prior to that (2011) only New York numbers.
https://web.archive.org/web/20130924200424/http://kmkhanlaw.com/Contact_Information.html
I checked the D.C. address given and it was sold May 7, 2013. Its a residential condo address. The current KMKhanLaw.com contact page does not list a D.C. number.
https://www.taxpayerservicecenter.com/RP_Detail.jsp?ssl=0540%20%20%20%202621
According to this site
he bought the D.C. condo in 2005. So he maintained that D.C. address from 2005 through 2013, during that time he briefly had a D.C. phone number for KMKhanLaw.com