The speed did vary in the UK, especially as some ring tone generators were motor driven, the two cadence was the normal for exchange lines. However, PABX lines were often given single cadence rings for internal calls and double for 'outside' line connections, thus making it easy to determine whether ones family or a fellow worker was calling, (choose which one to ignore).@Peasmould. You may have a point.
Believe the standard ringing pattern in the U.S. is/was something like two seconds of ringing, followed by four seconds of silence.
In the U.K., it is/was 0.4 seconds ringing, followed by 0.2 seconds silence, followed by another 0.4 seconds ringing, and then two seconds of silence.
Not sure if this ever differed for local calls/long distance calls.