One of you, may not have enabled iMessage for their Phone Number. Try sending to each others Apple ID. If that works the it indicates both of you needs to do the following.
To activate iMessage and FaceTime using your Phone number
First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you
If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)
If you can't turn on or sign in to iMessage or FaceTime - Apple ...
*** You may not have an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.