The only way I know how to do this is using a service called Tropo or an Asterisk PBX.
See:
http://www.obitalk.com/forum/index.php?topic=5310.0I do not use this app. I just wanted to see if I could make this work because a lot of people have requested this function. The call is routed to Tropo, routed back to the OBi and then out a trunk. When routing the call back to the OBi, I had to use another service that was registered to the OBi to eliminate a no audio problem.
Tropo has 2 types of accounts, a free development account and a production account. Tropo allows free outbound landline calls, so you can call directly without routing the call back to the OBi. I don't know when they will consider your account production and cut you off. If you are calling a tollfree number, you can use a free SIP tollfree provider and I'm guessing they would be less likely to consider you production.
Tropo has redesigned their Website since I posted the above and creating an App might have changed slightly. The new Website doesn't work well with Internet Explorer, so I use Chrome.