The modem checks for a dial tone. The modem dials the phone number. The other modem answers the call and they both do a process called "handshaking". This is where the modems will attempt to communicate at the same speed, with the same protocols. They do this communication over what is called a Carrier Signal. Under most circumstances 56k modems will first try to negotiate a V.90 connection, then a K56Flex connection and if both those fail they will do V.34. The modems will also try to negotiate the highest speed possible and then go down to a speed that they can communicate at.

During the connection modems can and often will change speeds depending upon the phone line conditions. They do this through renegotiations and retraining. If modems renegotiate (also called fall-back and fall-forward) they simply will change speeds. If they retrain they will do a complete re-handshake and form a new speed. Most modems have a setting that tells them to do a full retrain after a certain number of renegotiations and to disconnect after a certain number of retrains. It is worth noting that modems are often poor at determining what speed they should communicate at to have a reliable connection.