This could be caused by a number of reasons.
- If you are using a Wi-Fi connection, make sure that you are not behind a firewall or that the firewall settings are allowing Silent Phone to function properly. You can find a list of the ports Silent Phone needs to work here. If you have access to a mobile data connection, switch off your Wi-Fi on your device and use your mobile data network; if Silent Phone works then you know that the problem is with your Wi-Fi.
- If you are using an iOS device, please make sure that Silent Phone has access to your device's microphone. You can check this by going to your device's Settings > Privacy > Microphone > Silent Phone and making sure that the switch is turned on (green).
If Silent Phone has access to your microphone but the other party cannot hear you, it could be a problem with your connection or the other person's connection, especially if one of you is behind a firewall. You can check which ports need to be open for Silent Phone to work here. Make sure those ports allow traffic both ways.