A professional intercom wired into your building is a good investment, but avoid the plug-in radio wave (RF) types that use the power lines to carry the message. Such are convenient, but NOT secure. Anyone with another unit or equipment who can plug into your side of the local power transformer can listen in on your private messages such as how much cash is in the box office; robbers would love to know that.
Also, sometimes such RF types can inadvertantly broadcast outside the building and some people with scanners can pick up the messages/calls. On some such RF units, it is possible for someone else to jam your system and even place their phone calls at your expense. Opt for a hard wired system even if it is more difficult to install.