Lan didn’t know Xinyi’s mobile phone number before Episode 400, so how did she send a text message to Xinyi?

Japan’s mobile phone text messages are different from those in China. Although this phone is still used, it is not a phone number, but an address specifically used for sending text messages. This SMS address needs to be applied for by the operator (or needs to be activated). That’s why they call mobile text messages “mobile emails”.

In the beginning (about 400 episodes ago), Lan and Shinichi used text messages to communicate.

Later, in a certain episode (I forgot which episode it was), Shinichi called Lan and finally said to Lan, "Idiot, look at the number displayed on the phone screen." Only then did Lan realize that when Shinichi called, he also told himself Shinichi's current phone number. Finally, he could not only contact him through text messages.