Believe it or not, there were places which used to have numbers such as 39F011 or 42F12. Geek-of-the-week award goes to anyone who can explain those!
It looks as though nobody is going to win the prize, so here's the explanation.
These numbers were used on some manual exchanges with those huge multi-way rural party lines. The first part is the actual line number, and the numbers following the "F" indicate the ringing code for the particular party. The first digit is the number of long rings, the second digit the number of short rings, third digit is long again, and so on.
So 42F12 means line 42, and the cadence for the party required is one long ring followed by two short rings. 42F11 would be the same line with a ring of long-short, 42F1 would be just a single long ring, and so on. 42F02 would be a short-short ring, since the zero indicates no long rings before the two short. 42F021 would be short-short-long, and so on.