We have used the Rest API successfully when using the Get method but are now working on XML response to a POST. The connection is successful, but the results are in a single 'output' field in CSV format which is truncated due to field length as we are reporting on multiple records.
The Post method template is similar as follows :
<?xml version='1.0' encoding='UTF-8'?>
<call method='exportData' callerName='a string that identifies your client application'>