process_ipn_return not being called

I am completing the development and testing of a MarketPress Payment Gateway plugin for using Touchnet Payment Systems UPAY as the payment services provider. I have all aspects of the process working except for receiving the notifications from TouchNet when the payment process is complete. I have extracted the $IPN_URL value to use for testing the callback process. To determine wheter the process_ipn_return() is being called, I add a wp_mail call that sends an email message to me when the call is made. The start of my function_ipn_return() is:

function process_ipn_return() {
global $mp;

wp_mail('','TouchNet process_ipn_return v5','Message body','Content-Type: text/html; FROM:');

The register call is:

mp_register_gateway_plugin( 'MP_Gateway_TouchNet', 'touchnet-payments', __('TouchNet Checkout', 'mp'), true );

Thanks for any help you can provide.

Mike Cagle