title: What is the trick used by some iphone sip apps to communicate with UDP only sip servers?
body: <p>There are some ios sip applications who are able to communicate with a UDP only SIP Server. </p>
  
  <p>As I know iOS allows only TCP connection to remain open in the background but most of the SIP providers are supporting only UDP.</p>
  
  <p>I have noticed that iOS application 3CXPhone has a "NAT helper mode" and it is able to keep the communication in background with a 3CX Phone system who is UDP only. Dose anyone know what trick do they use? I am developing an SIP app and I have to make it work for the UDP only SIP providers.</p>
  
  <p>I know there are multiple questions regarding UDP socket in background on SO but none of them has a useful answer or the solution proposed there dose not work anymore (starting from iOS 6).</p>
  
  <p>Until now I am aware of 2 possible solutions:
  1. Use some GPS events and during that events maintain the socket communication too. After that try to trick apple and get your app in the store.
  2. Use a SIP proxy in the middle (B2BUA). But in the 3CXPhone "NAT helper mode" I am not seeing any sip proxy configuration. </p>