Maybe, but 199 was free, and triggered a callback.

Dial 199, hang-up. Wait for the incoming call (phone may or may not actually ring - some had the ringer disabled, but you could still answer the call). Hook flash to get a new dial-tone, and make your free call. The payphone still thought the incoming call was active, so no charge, but the keypad was also disabled - thus the need for a DTFM tone generator to dial your number.