Form doesn't submitted
I have tested it in my module, i didn't get any issue. It worked fine for me.
The output i received after using print_r(); and die(); in my submit function is 

Array
(
    [zipcode] =&gt; 23423423
    [phonenumber] =&gt; 423423423
    [continue_submit] =&gt; Continue submit
    [form_build_id] =&gt; form-zVFjX4fMorTyOI6m3JE0SL3w0_rSCTsVzEho63Qb1u4
    [form_token] =&gt; Pg3erepuKQvtsCSQT-3G7FWLivsSWDH69P0Z3YfRD6M
    [form_id] =&gt; mhzipcode_storage_form
    [op] =&gt; Continue submit
)  


Just try it in a separate module where no extra code exists and then debug the issue. 
It can be conflict with some module or any other issue. Hope it will help.