In some cases, KakaoTalk may set your business' phone number to inactive, depending on your location and the level of activity associated with your account.
For example, if you are still in the process of configuring a KakaoTalk channel, and your business is located outside of South Korea, you may not be actively using KakaoTalk on a daily basis. If so, KakaoTalk may set the phone number associated with your account to inactive. Then, when attempting to send an AlimTalk message, you may receive an error like the one below:
CHANNEL_REJECT : The underlying channel reported: 3018: Unable to send messages (invalid or broken KakaoTalk user)
To address this, simply log on to your business' account and use the KakaoTalk app for a while. Then, KakaoTalk will reactivate your account. To ensure the system is properly synchronized, try sending a new message one day after your account is reactivated.
The Sinch Developer Documentation site contains comprehensive documentation that illustrates how Sinch supports KakaoTalk through the Conversation API.