sentences:  In periods of very low traffic, there would typically be fewer than eight call set-up attempts on the exchange in eight minutes and this would have prevented the above security system from working.The exchange was therefore provided with an automatic Test Call Unit, which originated a test call every 30 seconds.As well as thus enabling Call Control to detect eight failures in less than eight minutes (if all call-attempts were failing), the Test Call would raise its own prompt alarm if it detected 31 sequential call-attempt failures, indicating that neither security side of the exchange was able to connect calls.