I'm not sure if you can search through Line # within AXL; what I found that you can do is to use an "executeSQLQuery" command and build yourown SQL Query for what you need. In your case, to list all phone name (SEPXXXXXXXX) which its line starts with "1000", you will need to run this query:
SELECT device.name
FROM device
INNER JOIN devicenumplanmap ON devicenumplanmap.fkdevice=device.pkid
INNER JOIN numplan ON devicenumplanmap.fknumplan=numplan.pkid
WHERE numplan.dnorpattern LIKE '1000%'
The whole SOAP XML would be:
<axl:executeSQLQuery xmlns:axl="http://www.cisco.com/AXL/API/8.5" xsi:schemaLocation="http://www.cisco.com/AXL/API/8.5 axlsoap.xsd">
    <sql>SELECT device.name FROM device INNER JOIN devicenumplanmap ON devicenumplanmap.fkdevice=device.pkid INNER JOIN numplan ON devicenumplanmap.fknumplan=numplan.pkid WHERE numplan.dnorpattern LIKE '1000%'</sql>
</axl:executeSQLQuery>