SMS sender ID has some local regulations that need to be followed. So in India, the rule is pretty straightforward, SMS sender IDs would be of maximum 9 characters with the following format:
XX-XXXXXX
Here, the 1st character would be the identity of the telecom operator who is sending the message, the 2nd character would be telecom circle. There is a fixed identifier that the Indian telecom regulation has provided for every telecom operator and circle.
The hyphen or dash remains as the 3rd character and the next 6 characters identifies the name of the sender of the SMS. So, the VM-XXXXXX in message sender IDs, the ‘V’ says it is Vodafone Group of Companies, ‘M’ is for Mumbai circle of operation.  The next characters could be anything like HDFCBK (for HDFC Bank) or FLPKRT (for Flipkart). Basically a VM-HDFCBK message sender ID means that the message is from HDFCBK which is actually originating from the Mumbai SMS operational circle for Vodafone telecom operator.