That would come from your Contacts app.
You choose what address is used under Settings > Safari > AutoFill. If you don't want the auto-fill, just turn it off. Otherwise pick the right contact and/or edit the one selected.
Some ideas. I'm just guessing here, so I won't guarantee results.
1) Make sure the desired phone number is the first listed. This may take some copy and pasting.
2) Change the label field to Main.
3) Create a new contact with just the info you want for auto fill, and select it under settings.
Good luck.