Sometimes it was possible to find a nearby local exchange which when the caller dialed the local code, then added 0, they would get free STD access. For those in the know, it was possible to modify the Post Office phone (preferably the 700 series) by fitting a suitable diode and push button. This allowed one to receive a call but prevent the charging relay from operating, hence not charging the calling party. The drawbacks were that the caller only got five or six minutes before the system would drop the call thinking it had not been completed. It was also advisable to contact the recipient to ensure he was expecting the caller before making the call.