Wireless Etiquette: A Guide to the Changing World of Instant Communication, Peter Laufer, 1999, pages 39-40:

The wireless as leash
    One of the strange sociological manifestations of mobile telephone technology is that once friends and colleagues know you carry a mobile phone, they expect to be able to get in touch with you whenever they wish and often feel insulted if you are not responsive to their rings.
"I used to date a girl who would get noticeably upset when she couldn't reach me right away on my wireless," is the lament from one frustrated boyfriend. "This was a problem. I did not buy the phone to be on a wireless leash, though she seemed to think of it that way."

    When calling wireless phones it's prudent to take into account that the friend or business associate we're calling might be in the bathroom, driving in dangerous traffic, in a crucial meeting, walking--hands full of groceries--or just ignoring the phone in favor of a gorgeous sunset.
    If the person on the other end of the connection is using GSM technology, there is another factor to take into account before you dial: what time it is where that phone is about to ring. GSM allows a phone company subscriber to use a mobile phone all over the world without changing phone numbers. That means you could be dialing a number you think is local because it shares your area code, but the phone rings half a world and a dozen time zones away, waking your poor jet-lagged friend from a deep sleep in a faraway hotel. If you do make that mistake one thing is certain to occur next no matter how hard you try to avoid it. After you apologize, you will invariably say, "What time is it there?" followed by a query about the weather.
    Another courtesy worth extending to those you call on a wireless phone is a brief, not lengthy, conversation. In most parts of the world the calling party pays for any calls to wireless phones. That means that the mobile user is not concerned with the length of the received call because no extra charges are incurred by answering the mobile phone. But in the United States and Canada, the mobile phone users generally pay for the air time whether they make or receive the call. Consequently, it is decent etiquette to keep your calls brief unless you're told not to worry about the charges.