No, UCx does not allow you to get an external line's dial tone. UCx works as a PBX - it selects a route for outbound calls based on the dialed digits and the phone's context (call restrictions). Trunks are then used based on the selected route's configuration.
Even though it would be possible to create a configuration that would allow a user to get an analog line's dial tone, I would strongly advise against that idea. The reason is that for such calls, the user would be:
- bypassing call restrictions
- bypassing call routing
- bypassing call tracking (no useful information for outbound calls in CDR)
- losing the ability to redial or review call log on the phone
etc.