I gather from your post that you are stationed overseas and only have access to a wireless device for contacting USAA.
HERE ARE SOME THINGS YOU CAN DO:
1. Visit the International Contact Information webpage found here for information on how to contact USAA from an overseas location.
2. You can e-mail USAA (see here) and have them CONTACT YOU (i.e. call you back). You could also ask about using software like Skype.
3. If you have another family member (located stateside) linked to your USAA Account then maybe they could call USAA on your behalf.
4. Avoid using overseas WIFI unless you are sure of their security.
I hope my suggestions help you.
Helpplease, Thank you for your comment. The two previous comments are good options for connecting with USAA while you are without a hard line phone. Unfortunately, I'm not able to solve customer service questions here in the community. Please contact a member representative using the CONTACT US button on usaa.com or by calling 210-531-8722 or 1-800-531-8722 when you are able. Thank you.