This is happens when a feedback loop happens between the webapp and a mobile number via the gateway due to autoreplies from the webapp.
See the Github Issue.
Solution: Add the offending number(e.g 800 or SAFARICOM) to the Outgoing Deny List in the webapp's app_settings configuration file.
"multipart_sms_limit": 10,
"outgoing_deny_list": "800, SAFARICOM",
"contact_summary": ""