Why an intercom ? Baddies ring them all till they find a domestic they can bull**** into opening the gate. Code reader that you change MONTHLY!!
It would depend on the physical location of it, but I would look at how it can be mounted to make it more difficult to steal and at the same time look at tamper protection linked to an alarm system. Difficult to be more specific though without seeing it physically. CCTV may act as a deterrent as well.
You can try and contact the manufacturers or distributors of the intercom and see if they make any anti-theft brackets for it. Or ask your security company about seismic sensors to trigger an alarm if the intercom is abused. It may take a bit of work to eliminate false alarms.
Another option is a CCTV camera with video analytics to trigger an alarm if someone is loitering around the intercom for more than a certain amount of time. This might get pricey though (although you can get a free version of software if it is just a few cameras) and might be difficult to differentiate between a genuine visitor who is just delayed at the gate and someone tampering with the intercom. An IP Camera, mounted high up and inside the property, linked to a PC running the free version of Axxon NEXT (www.axxonsoft.com), configured to alert on loitering at certain times of day (or evening) might work.
And This.Mount intercom inside the property on a pole, accessed through a small square hole in the boundary wall.