Hello - nice to see you back!!
In my past jobs, we had a full time receptionist but like you appear to do, had a rota to cover her lunch break. This worked well. If someone due to cover was off, someone else stepped in. Everyone was quite easy going about doing extra covers but all made sure someone was not doing more than was fair.
I think a bell or similar is useful for when she may be in the next office for a short time. From the customer point of view, I think it looks professional knowing they won't be expected to stand there with no means of attracting attention. We had a proper bell that sat on a velvet stand rather than a 'doorbell' type - looked more attractive.
If she needed to nip to the restroom in between beaks, she would simply phone someone to come and sit at reception. Nobody minded, in fact, sometimes it was a welcome break from our desks for 5 mins
Hope this helps
