Calling and Shanten Count
Shanten count is a measure of how close your hand is to winning,
and it is a very important concept.
Here, let us look at the validity of calling by focusing on shanten count.
Calls That Do Not Change Shanten Count
Calling means giving up one tsumo,
so if you call, your hand should actually move forward.
Example 1
Chi 
If you pon or here,
the hand still remains one-shanten, so it cannot really be called progress.
So as a basic rule,
“Calls that do not change shanten count are forbidden.”
Exception 1
If the hand is already in tenpai,
and calling improves the wait,
or calling increases the hand's value,
then you should call.
Example 2
Chi 
With a hand like Example 2, if or appears,
you should call and change the wait.
And the pon on or the chi on
can also be correct in some specific situations.
Exception 2
There is also the case where shanten count does not change,
but calling still makes the hand much easier to win.
In practice, this means hands where yaku such as Tanyao or Toitoi are involved.
Example 3
Dora
In Example 3, compared with insisting on a closed riichi hand,
opening the hand and aiming for Tanyao is much easier to win with.
So even though the shanten count does not change, calling is completely fine.
Chi
Example 4
Example 4 is a chiitoitsu one-shanten hand,
but as soon as a pon-able tile appears, you should pon it and head toward Toitoi.
That is because Toitoi is dramatically easier to win than Chiitoitsu in this case.
Number of Open Melds and Shanten Count
The more you call,
the higher your risk of dealing in becomes,
and the more limited your truly useful tiles become.
So when your hand is still very scattered, opening it aggressively is not efficient.
Example 5
Chi 
Pon 
Pon 
Hands like Example 5, where a beginner keeps calling and calling but the hand is still scattered, are extremely common.
Once you have made three calls, you should already be in tenpai.
If you are still only two-shanten, that is not even worth discussing.
before
Pon Pon
If the hand looked like this before the call,
then even if the upper player discards , you should still hold back.
The shape is too painful, and even after calling the hand is still unlikely to win,
so it is better to prepare for someone else's riichi.
As a rough benchmark:
Three calls: you should already be in tenpai; if you are only one-shanten, it should at least be a mangan-class hand such as Chinitsu
Two calls: you should be at least one-shanten; if you are still two-shanten, then the shape must be very strong or the hand should be worth at least 3900
If you set this kind of threshold for yourself,
the precision of your calling decisions will improve a lot.
Example 6
Dora
With a hand like this, even if you call on through ,
the hand is still only two-shanten afterward, and it is cheap.
That cannot be called a valid opening decision.
If it were a kanchan call on , then at least it becomes somewhat discussable:
Dora
Original Japanese page: http://beginners.biz/naki/naki07.html