One way to do that is to place emergency outdoor wireless intercoms at strategic areas around a construction site. You have a couple of different technologies to choose from for these intercoms. One uses standard two-way radio technology to call two-way radios, and the other cellular network technology to call your cell phone or landline phone. These intercoms are housed in vandal- and weather-proof housings and they have a button workers can press to call for assistance.
You can mount these intercoms using low-cost poles you can get at any building supply store such as The Home Depot, and they can be easily moved between job sites.
Since the system is temporary you can just mount them to standard eight foot galvanized, 2 3/8 inch fence pole with a decorative end cap on top. If you don't have power nearby, you can use a solar power system at the top of the pole.
Since eight foot is a good height that lets you install the antenna and solar panel up out of the reach of people, you still need a way to attach the pole to the ground without taking away any of it's length. You can also purchase a two inch piece of electrical conduit that the fence pole will easily slide over and then you can bolt the fence pole to the conduit after you set the conduit in concrete.
Mounting the equipment this way lets you quickly move the it to another construction site without having to remount the equipment to a new pole. When the project is done, you dig up the conduit and throw it away and you save the fence pole and equipment for the next event.
If you have existing licensed two-way radios, you can get models that will work with those. Otherwise you can use unlicensed MURS radios. One problem with using licensed two-way radios is that if you are moving around to different locales, that makes it harder to get a license since getting a license requires a 'frequency coordinator' who checks for existing two-way radio users in an area before a frequency can be assigned to the license. They want to make sure your radios don't interfere with existing license holders. If you are moving around that's virtually impossible to do.
The solution to this problem is to use unlicensed MURS business-class two way radios. There are five frequencies in the 150 MHz radio spectrum that are called the MURS service. MURS stands for Multi-Use Radio Service. This service was created in 2000 for use in the United States. MURS is a short range (can be several miles) service that uses the VHF (Very High Frequency) radio spectrum. The FCC does not require users of products for these bands to be licensed.
If you would rather have the construction intercoms calling your cell phone, you can get a version that works on AT&T or T-Mobile that dial out any three phone numbers you want. When someone presses the button, you'll be talking to them on your phone. This eliminates workers having to use their private cell phones to call you, or worse, having to come find you. All these units have a relay you can activate remotely that you could also use to unlock a door or gate, activate a siren, or for any application that requires a switch activation.
Either solution increases productivity and safety on the job site.