電話 and 伝言: calling and leaving a message
Distinguish the communication medium from the message to be passed on.
Making a call
電話 (でんわ) can mean a telephone or telephone contact. 「友人に電話をかけました」 means calling a friend.
「電話をかけましたが、誰も出ませんでした」 says a call was made but nobody answered. Dialing does not establish a conversation.
A message to pass on
伝言 (でんごん) means a message left for someone or passed through another person. 「伝言をお願いします」 asks someone to relay a message.
「母からの伝言を読みました」 says the speaker read a message from their mother. 伝言 is not restricted to spoken messages.
Who performs the action?
「帰ったら電話してくださいと伝言を残しました」 says the speaker left a request to call upon returning. The intended caller is the message recipient.
「伝言を伝えました」 reports relaying the message. Whether its request was carried out is separate information.
Does making a phone call establish a conversation with the other person?
Show the answer and why
No. They may not have answered, so successful conversation requires separate information.