https://en.wikipedia.org/wiki/Multimedia_Messaging_Service

Here's the technical spec, which manages to combine the single bit level detail of old school telco with the enterprisey goodness of SOAP.

http://www.qtc.jp/3GPP/Specs/23140-6g0.pdf

For example, this is how you say "OK" in MM7:

    HTTP/1.1 200 OK
    Content-Type: text/xml; charset="utf-8"
    Content-Length: nnnn
    <?xml version="1.0" ?>
    <env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
    <env:Header>
      <mm7:TransactionID xmlns:mm7="http://www.3gpp.org/ftp/Specs/archive/23_series/23.140/schema/REL-5-MM7-1-3" env:mustUnderstand="1"> vas00001-sub</mm7:TransactionID>
    </env:Header>
    <env:Body>
    <SubmitRsp xmlns="http://www.3gpp.org/ftp/Specs/archive/23_series/23.140/schema/REL-5-MM7-1-3">
      <MM7Version>5.6.0</MM7Version><Status> 
      <StatusCode>1000</StatusCode> 
      <StatusText>Success</StatusText></Status> 
      <MessageID>041502073667</MessageID>
    </SubmitRsp> 
    </env:Body></env:Envelope>

Your blog post above was also a good read. Thanks.