@RestResource(urlMapping='/jtelACD/getCallerInformation')
global with sharing class JTELACD_GetCallerInformation {
    @HttpGet
    global static String doGet() {
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;

        // This is how to access URLParams if required
        // List<String> URLParams = req.requestURI.split('/');
        // Example: ["","jtelACD","getCallerInformation"]

        // Extract the request parameters
        String CallerID = req.params.get('CallerID');
        String ServiceNumber = req.params.get('ServiceNumber');
        String ServiceName = req.params.get('ServiceName');
        
        // Perform SOSL Query over contacts and leads, searching for the phone number with *wildcards*
        String soslSearchString = '*' + CallerID + '*';
        List<List<SObject>> searchList = [FIND :soslSearchString IN ALL FIELDS
                                          RETURNING 
                                          Contact,
                                          Lead];        
        
        List<Contact> contacts = ( (List<Contact>) searchList[0] );
        List<Lead> leads       = ( (List<Lead>) searchList[1] );
        
        // Multple hits?
        if( contacts.size() + leads.size() > 1 )
        {
            // Multiple hits
            res.statusCode = 300;
            return null;
        }
        
        if( contacts.size() > 0 )
        {
            // Query for exact contact
            String Id = (String) contacts.get(0).get('Id');
            Set<String> fieldSet = schema.describeSObjects(new String[] { 'Contact' } )[0].fields.getMap().keyset();
            List<String> fieldList = new List<String>( fieldSet );
            String query = 'SELECT ' + String.join( fieldList, ',' ) + ' FROM Contact WHERE Id = \'' + Id + '\'';
            Contact contact = Database.query( query );
            // 200 OK will be returned by default
            return JSON.serialize( contact );
        }
        
        if( leads.size() > 0 )
        {
            // Query for exact lead
            String Id = (String) leads.get(0).get('Id');
            Set<String> fieldSet = schema.describeSObjects(new String[] { 'Lead' } )[0].fields.getMap().keyset();
            List<String> fieldList = new List<String>( fieldSet );
            String query = 'SELECT ' + String.join( fieldList, ',' ) + ' FROM Lead WHERE Id = \'' + Id + '\'';
            Lead lead = Database.query( query );
            // 200 OK will be returned by default
            return JSON.serialize( lead );
        }

        // Nothing found
        res.statusCode = 404;
        return null;
    }
}

curl.exe --silent -i -X GET --header "Authorization: Bearer <OAUTH_TOKEN>" --header "Connection: Close" "https://<SALES_FORCE_INSTANCE_URL>/services/apexrest/jtelACD/getCallerInformation?CallerID=4989461495000&ServiceNumber=4980012345678&ServiceName=test"