If you come from kamailio and transfer your setup to FreeSWITCH as SBC you can run into trouble cos kamailio is not case sensitive but FS is.

So sip:user1@FS-Box is not the same as sip:UsEr1@FS-Box.
Here is a solution to fix that and make FreeSWITCH not case sensitive.

First your directory/default/user1.xml needs to force the sipusername to lowercase. sip-force-user did the trick for us. Here an Example:

<include>
  <user id="user1">
    <params>
      <param name="password" value="secretpass"/>
    </params>
    <variables>
      <variable name="sip-force-user" value=user1"/>
      <variable name="toll_allow" value="domestic,international,local"/>
      <variable name="accountcode" value="user1"/>
      <variable name="user_context" value="default"/>
      <variable name="effective_caller_id_name" value="user1/>
      <variable name="effective_caller_id_number" value="user1"/>
      <variable name="outbound_caller_id_name" value="$${outbound_caller_name}"/>
      <variable name="outbound_caller_id_number" value="$${outbound_caller_id}"/>
    </variables>
  </user>
</include>

Then your dialplan extension should match User1 or user1 or UsEr1 – (?i) mades the condition match incasesensitve for you. For example dialplan/default/user1.xml. Hardwired is the lowercase dialed_extension – so it matches with the lowercase registration.

<extension name="user1">
    <condition field="destination_number" expression="^(?i)(user1)$">
        <action application="export" data="dialed_extension=user1"/>
        <action application="set" data="ringback=${de-ring}"/>
        <action application="set" data="transfer_ringback=$${de-ring}"/>
        <action application="set" data="hangup_after_bridge=true"/>
        <action application="set" data="continue_on_fail=false"/>
        <action application="bridge" data="sofia/external/${dialed_extension}%${domain_name}"/> 
      </condition>
</extension>

Thats all. Alternativ you can use in the sip_profile the option
<param name=“inbound-reg-force-matching-username“ value=“true“/>
That force Contact must match Username on register. Tested with FS 1.2.1