T-Mobile is offering its free Bobsled VoIP service for iPhone and iPad users in the US, even though it doesn't sell the Apple devices.

The operator is also expanding its free voice-over-IP service to Android phones. Users with the app can make calls to mobile or landline phones in the US, Canada and Puerto Rico without incurring charges. Users can make the calls over cellular data networks, which typically charge for the data usage, or over Wi-Fi, which could be free.

The new services are an expansion of the Bobsled service that T-Mobile first launched early last year. Initially, it let people make calls only to Facebook friends from PCs. In October, it began letting people make calls from Facebook to mobile devices and make calls from PCs to US mobile and landline phones.

T-Mobile is also separately launching a Bobsled Messaging app for Android phones, with plans to also offer it for iPhones and iPads later this year. That app lets users send group text and multimedia messages.

Key to both apps is that they are tied to a user's mobile phone number. That means a user can download Bobsled Messaging on their Android tablet and tie the app to their mobile phone number, so that when they send a message, the recipient will see that the message is coming from the sender's regular phone number.

Users will find some benefits to downloading the app on multiple devices. For example, a user can read conversations on their tablet that were started on the phone. They'll also be able to access their contacts list from the app on both devices.

In the future, T-Mobile plans to start issuing phone numbers along with Bobsled. That means someone with a Wi-Fi-only iPad, for example, could attach a phone number to the tablet that friends would recognise when the user made calls and sent messages from Bobsled.

Brad Duea, senior vice president of marketing for T-Mobile, said that T-Mobile had a number of reasons for wanting to get into the VoIP business. VoIP traffic is only expected to grow, he said. "It might cannibalise some revenue, but we think it's a much greater opportunity. We don't have a fixed-line business to protect. So we're saying, 'cut the cord, here's how we can empower you,'" he said.

Some other mobile service providers that have landline businesses are often less keen to promote VoIP because it competes with traditional landline phone service.

For now, T-Mobile isn't saying much about how it will make money from the service. It plans to bring some new business models to the market, including ad-supported services and contextual advertising, he said. The company will talk more about that in a couple of months, he said.

The apps are available for download now from the Android and iPhone app stores.