procure low cost gps/gprs vehicle tracker

We have tested a number of trackers from China and have had stability issues or compatibility issues with all of them.? By compatibility I mean that some providers have even provided us their hardware protocol document and expected us to code the trackers ourselves - we do need this, just the commands on how to use the trackers from a web-based system. We just need someone to find a low cost tracker that can provide? GPS? tracking via sms&gprs, compatible with spanish/european telco provider networks (movistar or vodafone). Our developer will be integrating the tacker - he has provided the attached document for reference. Regards, Ferrino