Though more expensive than an OBi (in some cases much more), there are a variety of devices that address this issue and could be connected to your OBi.
You can find GSM-to-VoIP gateways
http://www.voip-info.org/wiki/view/VOIP+GSM+Gateways (for single channel, PORTech is popular), GSM-to-FXS (that you'd connect to the Line port of the OBi; they usually have an FXO pass through that permits a landline to also be connected), and Bluetooth-to-FXS (that you'd use together with an old cell phone, left connected to its charger).
You may find a used device at a good price that meets your needs.