If you have a firewall, it may not allow our services.
Configure the Needed Firewall Rules
One of your network devices (Firewall and/or Router) has an active SIP ALG.
SIP ALG stands for Application Layer Gateway.
It inspects the VoIP traffic and may modify the SIP packets.
Deactive the SIP ALG or configuration your VoIP-Devices to use TLS (if supported)
You have more than one Internet connection.
Your VoIP-Device/Client has to send the Voice (RTP) through the same Internet connection as the Signalling (SIP).
You have activated sRTP (encrypted Voice) on your VoIP-Device/Client.
Both sides (VoIP-Device/Client and SIP Line) have to be configured for sRTP.
Your telephone receiver or microphone is muted.
The volume of your speaker is low or muted.
Check your VoIP-Device/Client.