Thanks Joseph.
Secondary line is enabled for SMS. In fact for some of the Contacts, that is the default line.
For this particular Contact, I had initially set Primary as the default line. All calls/SMS were hence through Primary line. Later on I changed the default line to Secondary for this particular contact. But irrespective of default, all SMS communication still happens via primary line only because that was te line which was used for the last communication. I have noticed this before too for other contacts and the line gets changed if the contact replies to my secondary number and then all communication henceforth would happen only on the secondary line, there seems to be no option for me to choose the number to use for SMS communication and it always is the "Last Used Line".
Section in the document: "Send messages with iMessage and SMS/MMS"
I even tried this for the existing contact to use the new message function. I could select Secondary but the moment the contact was selected, the test message was sent through the Primary line.
Regards
Mani