The topic of this final labor was to explain in more detail what the carrier selection and pre-selection service means, ie what are all the possibilities for users to achieve voice telecommunication traffic and to detail the most commonly used implementation methods from the signaling aspect. Users, if they wish, have the option to select the carrier through which they want to make a call for each call, and this is enabled by the carrier selection service. However, if they decide that they do not want to enter the four-digit call routing code all the time, they can choose carrier pre-selection service and this entry will be performed for them by the exchange. To establish a call, the signal passes through several access points in the network and through several protocols. The most common implementation of calls is through ISUP and SIP protocols. Both protocols have their own signaling messages with which we can monitor the entire call, from picking up the handset to hanging up, and they help us see if the call went well or if there was a problem somewhere. If a problem occurs the protocols will send an error message with a specific label that is well known and defined in those protocols. With the increased demand for telephones and mobile phones and their rapid development, protocols had to be adapted and developed, so that today we are able to communicate with several different devices at the same time in the same voice channel, which was not the case a long time ago.