Deutsche Telekom has launched Wi-Fi calling in Germany for its customers with the Samsung Galaxy S6 and S6 Edge handsets.
WLAN Call is a free add-on to customers who have a VoLTE-enabled contract. Calls and texts made while a customer is abroad cost the same amount as within Germany.
The service is native to the smartphone and requires no specific app to be downloaded although consumers need to ensure their handset software is up to date. Deutsche Telekom said it was planning to bring the service to additional handsets in future.
A spokesperson said: "WLAN Call is of particular benefit in places that for construction reasons don't have sufficient mobile reception, but do have WiFi. Typical such places are underground garages or basements. In principle, all public and private WiFi networks can be used for WLAN Call. It requires a bandwidth of at least 100KB.
"Calls in Germany will be transferred to the LTE network without interruption when there is sufficient signal strength."
While the tech has been seen as a means of overcoming poor coverage indoors, operators have struggled with handover between Wi-Fi and LTE networks.
In spite of these concerns, a report from Deloitte earlier this year said both VoLTE and VoWiFi services will double during 2016 to 300 million customers.