Method for feeding call progress tones into conference connections

In a method for feeding call progress tones into conference connections in a pulse code modulation (PCM) time-division multiplex switching system, in which PCM code words are repetitively supplied by conference subscribers, sum code words are formed from the PCM code words and said PCM code words are intermediately stored in one half of a first conference memory, in which a plurality of different partial sum code words are formed by subtracting from a sum code word a respective different one of the supplied PCM code words, said partial sum code words being intermediately stored in a second conference memory before transmission of those partial sum code words to each subscriber, such partial sum code word containing only the code words supplied by the other conference subscribers, and in which the above steps are performed for subsequently-arriving PCM words with respect to a second half of the first conference memory and the conference memory halves alternately so perform the above steps at each new receipt of PCM code words, the improvement comprising the steps of:

• in addition to forming the sum code words, forming call progress tone sum code words, by adding code words stored in the memory halves to respective call progress tone code words;
• intermediately storing the sum code words and respective call progress tone sum code words;
• and
• selectively forming partial sum code words by subtracting a PCM code word pertaining to a particular subscriber from either the intermediately stored sum code words or the respective call progress tone sum code words on the basis of call progress criteria.