There is a very complicated etiquette when it comes to contacting Koreans by phone or email which leads a lot of foreigners perplexed.
By phone, Koreans seldom pick up phone calls they were not informed they were going to receive. That is, Koreans prefer contacting each other by text message first, and then call each other to complete the information they were exchanging by text message. Koreans only skip the text messaging part when the relationship with the caller is one of trust.
When receiving a phone call with an unidentified number, Koreans will pick up and wait for the person to talk first. If they recognize the voice and it happens to be someone they trust, they will talk. Otherwise, they will hang up.
Note that when Koreans absolutely want to talk with someone on the phone, they will repeatedly call the person until the person picks up. However, this is only done by people socially superior or equal. If Koreans consider that an appointment is important, they may also call repeatedly to make sure that the person is on its way.
Unless the person Koreans are talking to is a trusted person, they will tend to only exchange information regarding an appointment, and will refuse to talk about anything unrelated to the appointment time and place. They never choose to meet inside restaurants or cafes, as Koreans give appointments in front of subways stations or other public places.
Note finally that on the phone the expression ne is a very polite way of saying "goodbye" before hanging up. It is however considered impolite to hang up without saying anything.
When it comes to emails, Koreans address themselves differently depending on whether the person is a hierarchical superior, equal or inferior. When writing an email to a hierarchical superior, Koreans include a title in the subject, start the email by ___(name)께 and the question 안녕하십니까? They then write the body of the email and conclude with ___(writer's name) 올림
However, when addressing an equal or inferior, Koreans tend to be brief and concise. They do not include greetings, and may send all the information in the subject box without writing a single word in the body, or write a short paragraph with no punctuation using very brief information.
Koreans prefer contacting each other by phone and tend to use emails exclusively to send each other files or when they live in different countries. They may also email each other to ask each other favors which are not urgent.
Some Koreans prefer to negotiate small deals by email rather than by phone. If the offer is attractive or they have agree with it, they will reply, if it's not, they will not, pretending that they never received the email.
Finally, Koreans tend not to reply emails asking them for updates on their daily personal lives. They prefer using a box specially dedicated to that in their "cyworld" page.