Firstly,the default value for outside calls for that system is 9. Try it with 9 instead of 8. If it still doesn't work, the COS (class of service) for this phone needs to allow the call. Line redirect will not work unless the COS allows the call.
You will need administrative capabilities (password)to change this. There are several ways to do this, but the COS changes are made under the headings-admin-capabilities. The set must be programmed to use an existing cos allowing outcalls, or a new cos must be set up.
Hope that helps.