TELTEL PTY LTD’s VoIP services use an industry standard protocol called SIP. (See: What is SIP?)

Some Internet Service Providers (ISP’s) may block SIP traffic on their networks, which will prevent your VoIP service with us (and other SIP providers) from working.

A common symptom of service being blocked is errors such as:

• Error 503
• Could not connect to server
• Server unavailable

ISP’s known to block SIP:

• Optus 4G (see: Cannot connect over Optus 4G)
• Globe (in the Philippines)
• Belong (on some dynamic IP services)
• Most ISPs in China and Pakistan.
• All ISPs in the Arab Emirates

These are ISP’s that our support team has commonly encountered blocking SIP. This is by no means a complete list, but no other ISP in Australia is currently found to be blocking VoIP.

It is also common for SIP to be blocked on:

• Free WiFi access points
• Hotel WiFi

Resolution:

Unfortunately the only resolution to this issue is to ask the provider to stop blocking SIP on your service (which some will do) or to change to another internet service provider.

You may also choose to use a VPN service to circumvent the ISP’s blocking, although this may have a negative impact on call quality and is not a recommended solution.