I wonder if something like this could work?
https://www.amazon.ca/SadoTech-Flashing-Vibrating-Transmitter-Operating/dp/B00P90X4CM
It’s not specifically for dogs, but it’s stuck on his adhesive tape. So you can put it low to the ground and your dog and press it.
I haven't used a purpose built device for this... In my case I have a home full of Google home/nest speakers, and i've set it up so Dad can just say "Hey Google, call nrfx" and it rings directly to me.
There are a few bits of kit that have fobs you could use that are dependant on a land line or cell phone, then there is this, which I just found just now, but know absolutely nothing about:
Wireless Caregiver Call Button System - Medical Alert System for Seniors & Elderly Care
It claims a 1000ft range, but those kind of measurements are usually by line of site, as in, it will have much less range the more walls it needs to go through.
What about something like this? It can be set to vibrate and/or flash a light.