I am not sure that you can change what it matches to by default. I believe it is "helping" by looking for a match.
When you load lists, you should only load the columns you want to insert or update. Not only will this prevent you from having to deal with this, but the file will load much faster. If you find a certain field always matching phone number, you should try to exclude them from the upload file.
Also, make sure that your column headers are populated and match Marketo field names. If a blank column header is getting picked up, next time make sure that column has data for the system to read and ignore if it doesn't match a Marketo field name.
Elixiter wrote a great How To Here:
http://www.elixiter.com/importing-a-static-list-into-marketo/ It even includes a list of some of the most common field names across Marketo and SFDC.