After coming to the U.S. and activating my phone, I entered a Korean phone number in KakaoTalk, but sometimes I can't add friends.

When you first experience this, you might wonder, "Why isn't it working?" It can be a bit confusing. However, this situation is more common than you might think, and there are several reasons for it.

The most common reason is that the other person has disabled the option to add friends via phone number.

In KakaoTalk settings, there is an option called "Allow adding friends by phone number." Some people turn this off. If that's the case, even if I enter the phone number, it won't show up in my friend list. It's not that the number is wrong; it just doesn't appear due to the settings.

The second issue is the phone number format.

If the number is not saved correctly, KakaoTalk may not recognize it. For example, if it's a Korean number, it should typically be saved as

010-xxxx-xxxx or
+82 10 xxxx xxxx, including the country code.

The same goes for U.S. numbers. You may need to include +1 or enter the area code accurately for it to be recognized properly.

The third possibility is a sensitive issue: the other person may have already blocked me.

In such cases, searching by phone number or adding friends may not work properly. Of course, you can't conclude that someone has blocked you just based on this, but it is a situation that can occur.

The fourth reason is that the KakaoTalk account is not linked to the phone number.

Some people create their KakaoTalk accounts using only an email address, or they may have verified it with a different number they used in the past. In these cases, I won't be able to search for friends using the phone number I know.

The fifth issue is app synchronization problems. Sometimes, the KakaoTalk friend list may not update immediately. In such cases, you can go into settings and

Settings → Friends → Refresh Friend List

and pressing this once can often resolve the issue.

In summary, the reasons why you might not be able to add friends after entering a number in KakaoTalk usually fall into one of these five categories.

The other person has disabled adding friends by phone number.
The phone number format is incorrect.
There is a possibility of being blocked.
The KakaoTalk account is linked to a different number.
There is a synchronization issue with the friend list.

Most of the time, it's often a matter of settings or number format.

So rather than overthinking it, checking each one can often lead to a solution.