Fix a problem with Asterisk/Freepbx and an Axis IP Speaker C3003-E.

We have a PBX running on Asterisk and we have around 60 extensions configured. We use this in order to do remote voice alerts in softphones running on servers and some Axis IP speakers using SIP. We have many Axis speakers working and for this we had forwarded the necessary ports for them to work and we use the public IP address. All of the extensions are outside of the network our server is running. For the softphone we use Jitsi and we have preamplified speakers connected in the speaker plug of our sound card. So when from our office we dial an extension, we have Jitsi auto answer the call and then we speak and call talk and the sound is heard. When we use Jitsi, we use a VPN connexion (OpenVPN) so we use the VPN (local) ip address. But when we work with the Axis IP C3003-E speaker, we cannot use VPN so we use the public port of the server running asterisk and have the client open the outgoing and incomming ports needed (5060 tcp and 10000-20000 in udp). The problem we are experiencing with a customer is that the sound is not coming out of the Axis speaker and we have tried everything but nothing works :(