Know Your Switch
If you've ever wondered what kind of switch serves your exchange, you can just pick up your phone and listen. That's right - you can listen for particular sounds your line makes to find out whether you are on a #1 or #1A, a #5 ESS, or a DMS 100 switch. Also, when you make a call, you can tell what kind of switch you're calling.
For example, when calling from a #1 or #1A ESS, which is an electronic switch, you will notice two short "kerchunk" sounding clicks before the phone number you are calling begins to ring. If you are are calling a number that is on one of these switches, you will notice a click when the ringing line is picked up.
On digital switches such as the #5 ESS or the DMS 100, there are no clicks when calls are placed or when the other line picks up. However, there are ways to tell a #5 ESS from a DMS 100. In the New York Telephone network, if an exchange is served by a digital switch, you can dial that exchange plus the suffix "9901" and a recording will come on and tell you where the switch is located, what exchanges are on the switch, and what type of switch it is.
But there is another way to tell for those outside New York. For instance, a #5 ESS has a slight single is click before the dial tone when the phone is picked up. A DMS has no click before the Dalton.
Also, when you call a number that is on a #5 ESS, you will sometimes get a partial first ring. When calling a number that is on a DMS 100 switch, you will always get a full ring on the first ring. Also, the first ring on a DMS 100 tends to be slightly longer than on the #5 ESS.