asterisk users - Feb 2016 - D70 phone dials 800 when pressing Msgs button. How to change that ?

Hello,

I'm discovering Digium D70 phone (1.4.2 firmware) with asterisk 13.

When pressing phone's Msgs button, I see it's dialing extension 800 no
matter what I edited in my phone config file.
Relevant piece of phone config is:

<account server_uuid="0" index="0" status="1"
register="1" account_id="100"
username="dan" authname="dan" password="password"
passcode="100"
line_label="8004 Dan" caller_id="8004 Dan"
dial_plan="[0-8]xxxxx|911|9411|9611|9011xxx.T3|91xxxxxxxxxx|9[2-9]xxxxxx|*xx.T3|[0-8]xx.T3"
visual_voicemail="0" voicemail="sip:555 at 192.168.64.157"
outbound_proxy="192.168.64.157" outbound_port="5060"
conflict="replace">
            <host_primary server="192.168.64.157"
port="5060"
transport="udp" reregister="300" retry="25"
num_retries="5" />
            <permission id="record_own_calls" value="0"
/>
        </account>


I've tried in the above voicemail field, the following values:
sip:555 at 192.168.64.157
555

Suggestions ?

Best regards
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