Bill, I concur with your analysis and would venture to add that http://whocallsme.com/Phone-Number.aspx/02070971997 may bring you joy if the callers have omitted the leading zero from 020 7097 1997
However, as in all matters telephonic, I exercise extreme caution and on the first Sunday of each month I normally check http://whocallsme.com/Phone-Number.aspx/442070971997 and http://whocallsme.com/Phone-Number.aspx/0442070971997 and http://whocallsme.com/Phone-Number.aspx/00442070971997 as well. Just to be certain that it is indeed my fiance William calling.
That is unless I am convinced that he is indeed in Maine (Portland) and not London (England).