Probably the first place to start would be to figure out how to get a listing of the things that qualify as either standalone analog ports (e.g. an ATA) or gateway-attached ports (e.g. VG2xx/3xx) and then perform a 'get' request to enumerate the actual ports/lines configured on each.
Cisco DevNet: Administrative XML (AXL)
You can perhaps start with:
- listGateway : AXLSoap - listGateway
- listCiscoCatalyst600024PortFXSGateway : AXLSoap - listCiscoCatalyst600024PortFXSGateway
- listPhone : AXLSoap - listPhone
It may be tricky - and you may need to resort to using <executeSqlQuery> - to figure out how to search/walk the various types of things you may consider 'analog ports'...