You can shut down firewall by the following
iptables -I INPUT -j ACCEPT
You can try that sim first, if it does not work, buy another sim card
I had already shutdown the firewall on my working environnement. I’m working on a virtual machine which runs on windows 10, i also tried to shutdown windows 10 firewall, it is still not working. A little precision, the errno number that i obtain is 110, which correspond to “connection timed out”, which i think means that the mangoh receive the demand but doesn’t respond. I verify with the logreads that my tcpServer works, i indeed have the LE_INFO that said that the server is listening.
Suggest you trying on ubuntu instead of win10
Or you can capture wireshark log to see what happens in win10
I suggest you to use analogue phones since they are old school.