If I'm understanding your question, these are the steps I take when
debugging issues with MO messages possibly failing when executing the HTTP
GET/POST against the defined URL. If you've assigned values to the
*log-file* and *access-log* configuration settings for the *smsbox* and
*core* groups then you'll see log statements similar to the following to
help you trace
*core config group access-log setting*
Receive SMS [SMSC:<bind_name>] ...
*smsbox group log-file setting*
Starting to service...
*smsbox config group access-log setting*
SMS HTTP-request ...
These log statements will contain the HTTP response code. If you're heavily
exercising Kannel, it's possible the service responsible for handling the
HTTP requests is failing to keep up. Under the *smsbox* group you can
assign the *http-request-retry* configuration setting which takes care of
retrying the request n times if it fails.
Please note I am simply guessing here as I do not know the specifics
surrounding the failure. Hope this helps.