Frequently Asked Questions (FAQs):Firewall and NAT issues

Asterisk servers: 

Note:An Outbound proxy is mostly used in presence of a firewall/NAT to handle the signaling and media traffic across the firewall. Generally, if you have an outbound proxy and you are not using STUN or other firewall/NAT traversal mechanisms, you can use it. However, if you are using STUN or other firewall/NAT traversal tools, do not use an outbound proxy at the same time. If your firewall restricts incoming connections, you may have to use an Outbound Proxy to properly receive audio.

FreeVirtualNumbers.co.uk Outbound Proxy is:      
nat.freevirtualnumbers.co.uk:5082

STUN stands for Simple Traversal of UDP over NAT. It is a protocol which enables an IP phone to detect the presence and type of NAT behind which the phone is placed. An IP phone that supports STUN can intelligently modify the private IP address and port in its SIP/SDP message by using the NAT mapped public IP address and port through a series of STUN queries against a STUN server located on the public Internet. This will allow SIP signaling and RTP media to successfully traverse a NAT without requiring any configuration changes on the NAT. STUN presents a working solution for most NATs that are not symmetric NAT, e.g., most of the SOHO routers have non-symmetric NAT and in this case, it is OK to use STUN. However, STUN does NOT work with symmetric NAT and if your routers have built-in symmetric NAT, do not use STUN.

FreeVirtualNumbers STUN Server is:      stun.freevirtualnumbers.co.uk:3478

If you are restricting your firewall by incoming IP addresses you will also need to allow the following range of IP addresses through your firewall in order to be sure that your audio will be allowed between your devices and our servers:
193.111.200.0/23
213.166.5.134

If you have Multiple VoIP phones/adaptors behind your firewall you will
need to assign a unique Listening SIP Port and a unique Listening RTP Port for each Phone/adaptor.

Although we have taken the utmost care in compiling this information we cannot guarentee that the information will remain current and suggest that you check your manuals and search the internet for the latest information for the specific device you are using.

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