@dans...
Thanks for your suggestion. We have (had?) a local technician (company), but unfortunately the technician we utilized has "moved on to greener pastures" (ie, retired)....and locating one that works on older equipment has been difficult. Everyone wants to sell us new equipment, but that doesn't necessarily "fix the problem". We're still actively seeking a new technician....and will request they try your suggestion. I appreciate your input....thanks!

@Rather_Be_Fishing
Thank you also for your reply / suggestion. I might be able to try that myself (old POTS analog handset). I don't have a butt set (as @dans suggested), but I do have an old analog phone and will try that....and the wiring isn't too far removed from my abilities. I appreciate your insight.

Thank you both (again) for your suggestions.
With the recent coronavirus "shelter in place" ordinances in my area (DFW), our business has slowed and employees are now working off-site. I'm still engaged with daily activities at the office (in person), but haven't been able to reproduce the reported problem.

I'm going to use this quiet time to dig out an old POTS phone and prepare for when this issue resurfaces again.....and will report back to let the forum know (just for future reference).

Stay safe, stay connected....