Yes, you can use the Meraki MG41 as the primary connectivity for your VoIP system, even if you're using a non-Meraki PoE switch and a non-Meraki firewall.
The Meraki MG41 is a cellular gateway designed to provide primary or failover cellular connectivity. It's not dependent on the brand of the PoE switch or firewall in use, as its main purpose is to facilitate the internet connection through a cellular network.
Your VoIP system would connect to the network as usual, with the MG41 providing internet connectivity. The PoE switch would supply power and network connection to your VoIP devices, and the firewall would manage network security as usual.