I´m building a doorphone system for a small office.

For that reason I´m not  using a SIP server in the network, as I´ve built a peer to peer system over the LAN of the office. I´m using a Fanvil i30 doorphone as the main device at the entrance, and a Fanvil X1 IP Phone in every office. They are all configured with static IP´s and the system works well. 

Now I want to add the option to receive calls in a notebook or in an Android phone. For the PC I've tried Mizu and MicroSIP, and they work at least with minor problems. But when I try Mizu-droid in my Samsung S9 with Android 10, connected to a local WiFi, I can't receive calls. I install the app, skip the configuration of SIP server and credentials, and dial direcly the IP address of the mobile, from any of the Fanvil devices, but the mobile phone doesn't react, even if I have it unlocked and with the app open. However, I can call from mizudroid to the fanvil X1.

Thinking the problem could be specifically of my phone, I tried an old Galaxy tablet with Android 4, but it's not working neither.

Perhaps it's a matter of misconfiguration, if you can help I can send you logs, or more information if needed.

Thanks in advance.