Sync iPhone contacts to custom account

I have an iPhone 4. I have set up a custom email account on my phone. It is also set up on my Mac.

When I add contacts to my contacts on my Mac the contacts appear on my iPhone but if I create a contact on my iPhone then the contact doesn't sync back up to my mac.

It looks like the contacts are being added to the phone not to the account on the phone. Is there a way to specify they be added to the custom account rather than the phone?