In contacts I have home phones listed followed by IPhone number. Before IOS8 and subsequent updates I could pick the IPhone number and send a message with no problem. Now, however, when I know I've doubletapped on the IPhone number, the home number is what appears in the to-line. I can only remedy this by making sure that the IPhone number is the top choice of phones for a given contact. The same happens with addresses; if I doubletap on a secondary address, what gets put in is the first address on the list. People I've spoken to at Apple don't seem to know about his, but at least one other friend has had my same experience.
This has happened to me as well.
I had to delete the home number so I could send a message to the mobile number.