Troubleshooting Lync FOR Mobile client issue:

1. Web Services authenticationcheck. To quickly verify the web services URL authentication settings, use theLync Management Shell to run the following cmdlet.
Get-CsWebServicesConfiguration |fl  Verify the value for the UseWindowsAuth parameter is set to Negotiate.

2.LyncMobile network traffic to Lync Server is entirelyweb based, and therefore connects to the internal Lync Web Servicesthrough the reverse proxy that is located in the network perimeter.As such, Lync Mobile clients neverconnect to the Edge Server. All traffic goes through the reverse proxy.

3.Lync Mobile clients authenticate to Lync Server using NTLM authenticationonly specified in the Authorization field of the HTTPS request.

4.Verify that Authentication Delegation is verified on the reverse proxypublishing rule configuration. This must be set to No delegation, but clientmay authenticate directly. If the reverse proxy publishing rules are set to Nodelegate and client cannot authenticate directly, it fails to sign-in when itreaches the step to provide credentials to request a token after MEX retrieval.
Basically the iOS clients do not support the basic NTLM Authenticationmethod while Windows Phone and Android clients do.

Troubleshooting External LyncMobility Connectivity Issues Step-by-Step
http://blogs.technet.com/b/nexthop/archive/2012/02/21/troubleshooting-external-lync-mobility-connectivity-issues-step-by-step.aspx