These GSM watches are available as replacement for smartphones at very low-cost. Model names are G36 and Q50.
Features are
- speakerphone
- basic calls
- Internet based time
You can add them as classical extensions on your PBX. A parallel call will ring them together with your SIP phone. Your trunk's caller ID should be added as a contact number to the watch, to make the watch whitelist all forwarded incoming calls.
Watches can be configured easily via an Android app ("Setracker"). An additional SIM card with low-bandwidth always-on Internet is required.
Of course, it's great for kids, too!