The OBi cannot send out DTMF automatically, except on the Line port of an OBi110.
The attached phone may have speed dial buttons or a contact list that could do this. You could also automate two-stage dialing with e.g. a free PBXes account.
However, you can likely find an equal or better value from a SIP provider. Are you calling landlines (if so, which city)? or mobiles (if so, which carrier)? What rate are you paying now?
Some SIP providers with low rates to Serbia include CallWithUs, CheapVoIPInc and Localphone; all can function as calling cards as well. Even less expensive, though SIP only, is Voxbeam (another Localphone brand). Localphone offers a 5-minute free trial at signup, Voxbeam has a $1 free trial.