Oona Räisänen hat auf ihrem Blog eine extrem detailierte Analyse des ollen Modem-Sounds anhand der Wellenform-Visualisierung.
The first thing we hear in this example is a dial tone, the same tone you would hear when picking up your landline phone. The modem now knows it's connected to a phone line and can dial a number. The number is signaled to the network using Dual-Tone Multi-Frequency signaling, or DTMF, the same sounds a telephone makes when dialing a number.
The remote modem answers with a distinct tone that our calling modem can recognize. They then exchange short bursts of binary data to assess what kind of protocol is appropriate. This is called a V.8 bis transaction.
Now the modems must address the problem of echo suppression. When humans talk, only one of them is usually talking while the other one listens. The telephone network exploits this fact and temporarily silences the return channel to suppress any confusing echoes of the talker's own voice.