This is the 3rd party response, and as far as I understand is utf-8:
HTTP/1.1 100 Continue
HTTP/1.1 500 Internal Server Error
Server: gSOAP/2.7
Content-Type: text/xml; charset=utf-8
Content-Length: 576
Connection: close
<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:sgdata="urn:sgdata"><SOAP-ENV:Body><SOAP-ENV:Fault><faultcode>0</faultcode><faultstring>Exception</faultstring><detail>Error en multiSignedMessage_Init. Err=[SeguriSign] - Transacción de Autenticación - La información no es auténtica</detail></SOAP-ENV:Fault></SOAP-ENV:Body></SOAP-ENV:Envelope>