How to Launch Wi-Fi Calling (VoWiFi) on Your Network: an ePDG Deployment Guide for Mobile Operators

Wi-Fi Calling (VoWiFi) gives subscribers voice and SMS on their own number over any Wi-Fi network, with no separate app. For an operator it fills coverage gaps (remote households, high-rise buildings, large offices and malls), and for an MVNO it can lower traffic costs. The node that makes it possible is the ePDG.

This guide covers where to start, the order of work, and the integration points where an operator most often loses time. As a reference we use the MVNO Miatel, which in early 2025 became the first operator to launch Wi-Fi Calling commercially on VAS Experts' Stingray ePDG.

What you need before you deploy an ePDG

• An IMS core. VoWiFi, like VoLTE, is an IMS service. The ePDG gives secure access; the IMS core handles registration, call control and the voice service itself.
• An EPC with a PGW that supports the S2b interface towards the ePDG.
• A 3GPP AAA server and an HSS to authenticate subscribers by SIM (EAP-AKA).
• SIM profiles you can update with the IMS and ePDG settings.
• A target device list: the phone models you will support at launch.

You do not need to finish VoLTE first, but many operators do. Both services have similar requirements for information systems and deployment processes, so experience from VoLTE makes the VoWiFi rollout easier. That was the order at Miatel.

How the connection works

1. The phone connects to an available Wi-Fi network.
2. It builds an IPsec tunnel to the ePDG using IKEv2. The tunnel is transparent to the local access point.
3. The subscriber is authenticated by SIM against the operator's AAA server using EAP-AKA.
4. Traffic enters the core through the PGW, much like an LTE session, and voice reaches the IMS core the same way a VoLTE call does.

In 3GPP terms the main reference points are SWu (phone to ePDG), SWm (ePDG to AAA) and S2b (ePDG to PGW). IMS signalling does not end on the ePDG: it travels through the tunnel and the PGW. That is why VoWiFi integration is a question about the whole chain, not about the gateway alone.

A launch plan in seven steps

1. Set the scope. MNO or MVNO, which devices, which Wi-Fi access policy.
2. Check core readiness. IMS, PGW with S2b, AAA and HSS, and the policy and charging chain (PGW, PCRF).
3. Deploy the ePDG in a lab and integrate it. IKEv2/IPsec towards phones, EAP-AKA through AAA, S2b to the PGW. Test throughput and latency under encryption load early, because the node encrypts and decrypts every tunnel.
4. Agree on the ePDG domain and certificate. Phones need to find the gateway and trust it. If the domain has to be issued by an outside party, file the request first and run it in parallel with everything else.
5. Configure SIM profiles and phone carrier settings. The SIM has to carry the correct IMS settings, including the ePDG domain.
6. Test on real phones. Registration, a voice call, handover between Wi-Fi and LTE, and an emergency call, on every target model.
7. Pilot, then widen. Launch to a limited group, then extend the device list and the subscriber base.

Where integration most often goes wrong

• The IMS leg. Registration and call setup cross the ePDG, the PGW and the IMS core. Test the full path, not only the gateway.
• SIM and phone settings. Carrier settings depend on the device. Some models take the new domain from the SIM without trouble, others need a different approach, so plan device-by-device testing and do not assume one profile fits all.
• Domain and certificate. Both sit on the critical path of the first successful connection. Start them early.
• Handover. A call must survive the move from Wi-Fi to LTE and back. Test it with real phones, in both directions.
• Emergency calls. The call does not use the cellular network, but the subscriber must still reach local emergency services. Agree on the behaviour with your regulator and test it.
• Wi-Fi access policy. Decide whether you restrict connections to specific hotspots or leave the choice to the subscriber.
• Policy and charging. VoWiFi is a separate traffic type. Check that it passes your policy and charging rules in the same way as LTE sessions do.
• Encryption load. Size the ePDG for the number of simultaneous tunnels you expect, using test results on your own traffic profile.

Reference: a first commercial launch on an MVNO

At Miatel, VoLTE came first, in response to customer requests, and Wi-Fi Calling followed. The ePDG was integrated into the operator's infrastructure, a knowledge base was built for scaling the service, and the specifics of VoWiFi on virtual operator networks were studied. The domain request, the SIM configuration and the device list set the pace of the project, more than the gateway did. Miatel keeps extending the list of supported devices.

A checklist for your own launch

• The IMS core, PGW (S2b), AAA and HSS are ready, and each integration has a named owner.
• The ePDG domain is agreed, and any outside request is filed and tracked weekly.
• The ePDG certificate is validated on real devices, not only in the lab.
• The SIM profile carries the ePDG domain and is tested on every target phone model.
• The device list is fixed, and each model has been through the full test: registration, call, handover, emergency call.
• The Wi-Fi access policy (restricted hotspots or free choice) is decided.
• VoWiFi traffic is covered by your policy and charging rules.
• The ePDG is sized for the expected number of simultaneous tunnels.

Takeaway

Plan a VoWiFi launch around the whole chain (IMS, PGW, AAA, SIM) and the device list, not around the gateway alone. The gateway is the part you can verify in a lab. The phones, the SIM settings and the integrations are where a real launch is decided.

VAS Experts builds the Stingray ePDG for operators and MVNOs. Product details and datasheet: vasexperts.com/products/epdg. Reference launch: ePDG implementation at Miatel.