VoWiFiAlso known as Wi-Fi Calling, Voice over Wi-Fi

How VoWiFi works

Wi-Fi is treated by 3GPP as “untrusted non-3GPP access”. The phone authenticates with its SIM (EAP-AKA) and builds an IKEv2/IPsec tunnel to the operator’s evolved Packet Data Gateway (ePDG). Through that tunnel it registers with the IMS core using SIP and carries voice as RTP, with the same codecs and services as VoLTE.

• Same number, same bill: calls use the SIM’s identity and are billed as normal calls.
• Handover: many networks hand calls between Wi-Fi and VoLTE as coverage changes.
• Emergency calls need a registered address in some countries.

Requirements and limitations

• The operator must offer Wi-Fi Calling and enable it for that device model; many phones also need VoLTE enabled first.
• The Wi-Fi network must allow IPsec (UDP 500 and 4500) to the ePDG.
• Voice quality depends on the Wi-Fi link: latency, jitter and loss matter as they do for any VoIP call.
• Calling while roaming may be restricted by the operator.

VoWiFi for gateways in poor coverage

A phone used as a GSM gateway indoors or in a basement may have weak cellular signal but good Wi-Fi. With Wi-Fi Calling, calls still go through the operator’s network under its terms, with no bypass. GSMCalls works with the phone’s native calling, so Wi-Fi Calling can be used where the operator supports it; see VoLTE and Wi-Fi Calling for GSM gateways.