Its less than 0.5mm thick SIM sticker can be easily pasted on the chip of a physical SIM card.

The combined SIM card is slim enough to fit on an iPhone 6's SIM tray, and slides easily into the phone. It comes free with a $30 call and text credit purchase on Interfone's website.

First, register the SIM card sticker via the firm's My Interfone app with your Singapore phone number and e-mail address. This must be done before travelling. Additional credits starting from $25 can be topped up via the same app.

Interfone has its own published call, SMS and mobile data rates as it buys wholesale from third-party Jersey Telecom, which has tie-ups with some 100 telcos around the world including Singtel, AT&T and Hong Kong Telecoms.

It provides up to 90 per cent cheaper roaming rates. For instance, calls made or received in Malaysia, Australia, the US and Hong Kong cost 49 Singapore cents a minute using Interfone's solution.

Traditionally, calls cost at least $4 per minute to make - or at least $1.50 per minute to receive - while roaming in these destinations.

Each SMS is at a flat rate of 29 Singapore cents using Interfone's solution, as opposed to 90 cents in the US or $1 in Australia.

All calls and SMSes must be made from the My Interfone app to enjoy these rates. The amount spent on calls will be deducted from the purchased credits. But its data plans are expensive: a 24-hour unlimited data plan starts at $25. It will soon introduce a 30-day 1GB mobile data plan in Australia for $10.

Comparatively, M1's unlimited data plan in destinations like the United States, Thailand, Malaysia and South Korean is only $15 a day.

Irene Tham