A telephone conversation is a verbal communication between two or more people carried on by means of either mobile or landline telephones. It can be short and casual, or it can be longer and more formal. The format typically varies based on the relationship of the people talking on the phone.
If someone is calling a friend on a mobile phone, their conversation might be extremely casual. Because mobile phones typically show the names of people calling, when those people are known to the phone's owner, often the people on the call do not even use each other's names in greeting, and sometimes do not even say hello. Instead they answer the phone casually, and treat the conversation as if it were a brief text or other form of non-verbal communication.
However, if an executive's assistant is answering a phone call from his boss's colleague, the call is likely to sound more formal. Even if the assistant has caller ID to identify who is calling, he typically answers the phone with a formal greeting announcing his boss's name and possibly his own. The rest of the communication between the callers is also likely to be formal and polite.
While most telephone conversations occur between two people, both landline and mobile phones typically offer the option of conference calling, in which more than two people calling from different numbers can all speak to each other at once.