Do you want to offer a seamless GSM callback system. NovaTec can offer a solution based on a mobile phone software in conjunction with the GSM Gateway.
The user dials only the number he wants to call, the software in the mobile phone will route the call first to the GSM Gateway, the GSM Gateway will check the callers Access details, and will then call the caller back and automatically call the requested number (over GSM/IP or ISDN, whichever is the cheapest route) and then connect the two parties... this is completely seamless.
For more information drop me an email at anan@novatec.de