transfer_to_voicemail.xml

<extension name="transfer_to_voicemail">
    <condition field="destination_number" 
                expression="^transfer_to_voicemail$">
        <action application="sleep" data="1"/>
        <action application="set" data="continue_on_fail=true"/>
        <action application="read" data="3 5 {{ vm_id_prompt_file }} vmbox_id 30000 #"/>
        <action application="set" data="origination_cancel_key=#"/>
        <action application="transfer" data="validate_transfer_vbox_id"/>
    </condition>
</extension>

validate_transfer_vbox_id.xml

<extension name="validate_transfer_vbox_id">
    <condition field="destination_number" 
                expression="^validate_transfer_vbox_id$">
        {% if valid_vmbox_id %}
            <action application="sleep" data="1"/>
            <action application="set" data="call_allowed=false"/>
            <!--<action application="transfer" data="{{ vmbox_id }}"/>-->
            <action application="att_xfer" data="{{ vm_transfer_uri }}"/>
        {% else %}
            <action application="sleep" data="1"/>
            <action application="phrase" data="vm_invalid_id_prompt"/>
            <action application="transfer" data="transfer_to_voicemail"/>
        {% endif %}
    </condition>
</extension>