Telecommunication systems allow to make toll-free telephone calls between each other via the public telephone network. They can also transfer a telephone call from one telephone to another telephone within a company. With the use of ISDN, telephone conferences with several participants were also possible, telephone calls could be interrupted without ending the conversation. Furthermore, calls could be forwarded to other telephones within or outside the network. Telephone systems with ISDN standard also provided for the spreading of the waiting loop with background music. With the age of VoIP telephony, the demands and characteristics of telephone systems, which are now primarily software applications, also changed.