As i understand it if you employ someone as a lone worker then it is your responsibility to do a risk assessment to ensure safety of that worker - then put something in place - phone check in/supervisor visit/emergency procedure etc.
It may be that a low level risk site such as an office just needs the employee to have a mobile phone in case of emergency or access to that office's phone (with agreement from the client) however if its a factory with dangerous equipment or a building site then maybe a supervisor needs to pop in during each shift or you need a phone check in/out system so you know exactly when employee onsite.
Going even further there are electronic devices which track a persons movement or they can electronically clock in - there is also one company who will do the phone check ins for you (with a password system to ensure its the right person).
Its just a case of taking the time to think about the site, record the risks and implement something which for offices etc will take just a couple of minutes per site.