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We've just added support for TURN in the latest update to Groundwire. This article explains the basics on the purpose and how to use the feature.
TURN is a protocol that allows for an element behind NAT or a firewall to receive incoming data over tcp or udp connections. What's important for use with Groundwire is that it can resolve audio issues related to using Groundwire on networks that use symmetric NAT. Symmetric NAT is most often used on 3G networks, but could be used in other situations as well.
Many SIP providers don't handle symmetric NAT very well. Basically, STUN (Session Traversal Utilities for NAT) does not work over symmetric NAT, so any providers that require a STUN server will have audio issues on networks that use symmetric NAT. TURN provides a solution to that.
Using TURN in Groundwire
To use TURN, you'll first need a TURN Server to use. It is possible to run your own, but most basic users will probably want to use a service. You may be able to find others on your own, but here is a link to numb, a free TURN Server provided by Viagenie.
http://numb.viagenie.ca/
Once you sign up for a TURN Server, you will receive the server address and a username and password.
In Groundwire, tap on settings, then SIP Accounts, then your SIP Account. Tap on advanced settings. Enter the TURN Server in the field for STUN Server. Enter the username and password in the fields marked TURN Username and TURN Password.
The default setting for TURN Usage (auto) will normally be best. This means TURN will only be used in cases where symmetric NAT is detected. But some SIP providers may need to have TURN Usage set to always.
Note: Using TURN may cause latency (delay in audio). This usually means the TURN Server you are using is too far from you. For example, the numb server is located in Canada. This will probably be fine for users in the U.S. and Canada, but could cause significant delays for users in other parts of the world. You may just want to use numb to test if TURN resolves your issue, then try to find a TURN Server closer to you if it causes any delays in the audio.