Hey Donnel,
Rudi is probably right. There have been reports of problems with open location on the Applescript Users List.
So far the workarounds are:
AppleScript:
tell application "FaceTime"
open location "tel://1234567890"
end tell
Directly telling the app you want to open the URL seems to work.
Shell script:
open 'tel://1234567890'
Don't use a do shell script_ command in AppleScript when you can run it direct from an Execute a Shell Script action — unless of course it's part of a larger script.
-Chris