Why can't telecom mobile phones send messages to China Mobile and China Unicom, but they can send messages to telecom and receive messages from China Mobile and China Unicom?

Telecom mobile phones can send short messages whether they belong to this network or other networks. What you said can be sent to telecommunications, not to China Mobile and China Unicom. This phenomenon has not been found. Generally, you can receive questions that cannot be sent, but you can't send them selectively. You can try inserting your mobile phone card into someone else's telecom phone. If this is still the case, some functions of the mobile phone card are not enabled. If you can send it on someone else's mobile phone, it's a mobile phone problem.