Hello,
You should allow anonymous users to access the ServicePage.aspx by doing this in your web.config in the same directory as the ServicePage.aspx:
<location path="ServicePage.aspx">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>
For more information, I would like to point you to the following forum:
ASP.NET Webmethod - Authentication failed.
Best regards,
Michal Legen