Motorola Moto G — Ringer tone is continually muted

I have a Motorola Moto G 3td Generation mobile phone and sim provided to me by my company. The ringer volume keeps being muted, I think it is something to do with the 'do not disturb, function as I occasionally get a message to turn this off. However even if I try to turn this off and or turn up the volume within a few seconds or minutes it will be much reduced or muted again. I thought it was an issue with the actual phone so I got another phone and swapped the SIM card across and initially this seemed to work but then within a couple of days the problem retuned and now is bad as before.
Any help greatfully recieved as this is way beyond my company help desk.