Hi Mario…Which IOS release are you running on your 1750?
By your verbage “gatekeeper”, I’m assuming an IP gateway of some sort. Different gateways use resources differently. Some gateways are designed to treat voice traffic like streaming media (H.323), others use a routing protocol like MGCP, others use layer 2 protocols like Frame Relay for fragmentation and prioritizing traffic. Can you be more specific?
Also, the 1750 router is designed for small offices, and if you want to use it for large-scale applications (like prepaid calling cards) you will be over-tasking your hardware.