SIM800 EVB connection problem

Hello, I'm having an issue with a SIM800L GSM module, pictured above. I actually have two of these modules, and I'm able to make one working (I can send a simple SMS message to my phone). But when I swapped and used the second one into the same circuit, I can't get it to work.

I've checked: this problematic board is able to turn on and connect to a network. Its SIM card works, and when I try to send a message it or call its SIM (using my phone), its RNG led indicates it has received a message or an incoming call.

So the problem must be in the connection between my Arduino board and the GSM module. Do you guys have any suggestions on how I can fix this? Or should I just accept that fact that this GSM module is defective?

Edit: I added a photo of the circuit I used to test the two modules. Like I said above, one works just fine and can send an SMS message to my phone, but the second GSM module fails to do so.