It is possible to build a small unexpensive box for making and receiving GSM calls and SMS. … When travelling outside the country, it is interesting to be reachable and be able to perform outgoing calls through VoIP without paying roaming fees. A Raspberry Pi can be transformed into a cheap VoIP box that routes VoIP/GSM calls in both sides (outgoing and incoming calls) as well as SMS.

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Raspberry Pi B+ with Asterisk and chan_dongle on Arch Linux (AdeledA)