Outgoing Calls from Personal Cell Phones - How to mask number and not look like Spam?

Hey hey,

Prior to just going directly to Google, I thought I would start here.

My Devo team is asking if there's a way to mask their phone number on their personal cell phones when making outgoing calls to donors while remote. Unfortunately, we're still rocking an old PBX system so there's not VoIP solution at play here. Any *free* services I can direct to my users for this purpose? I don't think "Unknown Caller" will be a well-received solution.

Thanks!