VoIP Software with SIP Protocol Conversion

I'm looking for expertise in software-based GSM or CDMA VOIP solutions. The successful freelancer should provide a solution that includes:

- Strong skills in developing software for VOIP gateways
- Specific experience in Protocol Conversion
- Proven knowledge of SIP protocols

Understanding of other aspects such as call routing and billing systems could be beneficial, but the core requirement is a well-designed, reliable protocol conversion solution. First-hand experience in similar projects would be much appreciated.