My suggestion would be to write a [powershell?] script that would perform the action you needed and set a custom property; SWO could then use the custom properties to trigger other alerts.
e.g. dial the modem
if a reply then set a custom property to one value
if no reply then set a custom property to a different value
I posted a powershell script the other day to set a custom property from a syslog or snmpTrap; it should work from an advanced alert as well, all that would be needed would be a way to script up the phone call.
/RjL