This article will explain how to sync your 123-mail contacts with your iPhone.
Go to your mail accounts set up page and choose Other.
Under the contact section, select Add CardDAV account.
Now you must fill in your information. For the server details enter: dav.123-reg.co.uk. Your username and password will be the same as your 123-mail username and password. You can enter whatever description you want.
Click next to complete the operation.