Hi,
Your scenario will work with Hunt Group and Hunt Pilot feature. On Hunt Pilot, configure Alerting Name and ASCII Alerting Name as your 1800 number for identification, for example 1800-222-3333 (Finance Dept).

When someone calls this number, you can route this from Unity Connection Call Handler to CUCM Hunt Pilot. On the phone display it will look like this:

There may be other options/solutions but this one works fine.
Please rate helpful posts and if applicable mark "Accept as a Solution".
Thanks, Vaijanath S.