Singapore’s largest operator Singtel launched Wi-Fi calling, also known as voice over Wi-Fi, for its postpaid customers.

The free service, which allows customers to make HD quality voice calls and send SMS via any local Wi-Fi connection without using an app, is available on select iPhone, Samsung Galaxy and Sony Xperia smartphones. Singtel said additional models that support the service will be introduced in the coming weeks.

The calls supposedly switch seamlessly to its 4G network when users move out of Wi-Fi hotspot coverage. But calls and text messages made over Wi-Fi will use the talk time and SMS bundles of customers’ mobile plans.

The operator also introduced a mobile data add-on that triples customers’ data allowance for a flat fee of SGD9.90 ($7.27) per month.

The new DataX3 add-on is available to new and re-contracting customers with Combo 2 and above plans (2Gb to 12Gb of data) with a minimum subscription period of 24 months. The triple offer follows the launch of the DataX2 add-on earlier this year.