I don't actually think the "tick tick tick" noise on those ARF crossbar lines had anything to do with switching. As far as I know it was applied as a "progress tone" in the same way that the E10 digital switches apply http://www.eircomlab.com/audio/cpt.wav when there is any delay before a ring tone.

I would guess that the original reason for that tone's exsistance was to prevent phreaking of the system using MF tones in the days of in-band signaling.

When you were calling "digital - digital" the number usually just rang out immediately but when calling digital to ARF / ARK crossbar or to a mobile or abroad (e.g. to the UK to an old switch) the bebebebe tone was played down the line rather than letting you hear the actual signaling.

The switches still present that tone if there's any sort of delay in connecting the call which is almost exclusively when calling mobile phones while the network is hunting for the phone or occasionally if you're calling abroad, particularly to less developed countries.

Although, it can crop up if there's a major network problem or something weird like our local radio station gave out its reception number rather than it's competition line number and 40,000 callers all called a single local number.. lots of bebebebe tones [Linked Image]

In other countries if a switch is swamped it just won't give a dial tone at all. Here the switch will take the call, let you dial and then make you wait until it's ready to process the call.