Allow NTLM authentication
In order to access secured data in SharePoint, NTLM is required. As we cannot include the System.Web.Services assembly, there seems to be no way to perform NTLM.
Bart Read commented
It sounds to me like what you need is more like LDAP authentication to allow your users to use their domain credentials since you can't add (certainly non-Windows) mobile devices to your domain. Is this correct?