Following is a sample proxy service and fault handler sequence you can use as a starting point for handling faults when integrating with SMPP connector. Customize this sample to match your requirements.
Sample Proxy
<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse" name="SMPP_sampleProxy" transports="https,http" statistics="disable" trace="disable" startOnLoad="true">
<target>
<inSequence onError="faultHandlerSeq">
<init>
</init>
<filter source="$axis2:HTTP_SC" regex="^[^2][0-9][0-9]">
<then>
<switch source="$axis2:HTTP_SC">
<case regex="401">
<!--Fill with your error code value and expression-->
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression />
</case>
<case regex="422">
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression/>
</case>
<case regex="404">
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression/>
</case>
<case regex="403">
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression/>
</case>
<case regex="400">
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression/>
</case>
<case regex="500">
<property name="ERROR_CODE" value=""/>
<property name="ERROR_MESSAGE" value | expression/>
</case>
<default>
<property name="ERROR_CODE" expression="$axis2:HTTP_SC"/>
<property name="ERROR_MESSAGE" value | expression/>
</default>
</switch>
<sequence key="faultHandlerSeq" />
</then>
</filter>
<respond />
</inSequence>
<outSequence>
<send></send>
</outSequence>
</target>
</proxy>
faultHandlerSeq
<sequence xmlns="http://ws.apache.org/ns/synapse" name="faultHandlerSeq">
<property xmlns:ns="http://org.apache.synapse/xsd" name="contentTypeValue" expression="get-property('transport', 'Content-Type')"></property>
<filter xmlns:ns="http://org.apache.synapse/xsd" xpath="get-property('contentTypeValue') = 'application/json' or get-property('contentTypeValue') = 'text/json'">
<then>
<payloadFactory media-type="json">
<format> {"error_code":"$1", "error_message":"$2"}
</format>
<args>
<arg expression="get-property('ERROR_CODE')" evaluator="xml"></arg>
<arg expression="get-property('ERROR_MESSAGE')" evaluator="xml"></arg>
</args>
</payloadFactory>
<property name="messageType" value="application/json" scope="axis2"></property>
</then>
</filter>
<filter xmlns:ns="http://org.apache.synapse/xsd" xpath="get-property('contentTypeValue') = 'application/xml' or get-property('contentTypeValue') = 'text/xml'">
<then>
<payloadFactory media-type="xml">
<format>
<error_info>
<error_code>$1</error_code>
<error_message>$2</error_message>
</error_info>
</format>
<args>
<arg expression="get-property('ERROR_CODE')" evaluator="xml"></arg>
<arg expression="get-property('ERROR_MESSAGE')" evaluator="xml"></arg>
</args>
</payloadFactory>
<property name="messageType" value="text/xml" scope="axis2"></property>
</then>
</filter>
<respond></respond>
</sequence>