Regarding this query of yours, there are multiple VPN platforms or services available online that will be suitable for your solution. Mainly you want to have an encrypted tunnel connection with your calls. In that case, I just did a quick google search and came across this: 5 Best VPNs For VoIP Services (2022) | Why use a VoIP VPN? (proprivacy.com)
We cannot recommend you the exact VPN platform or service to use because mainly there will be multiple factors concerning that let's say, location, price, throughput, package and not limited to this.
But when it comes to our device it supports multiple VPN functionalities (OpenVPN, IPSec, L2TP, PPTP, WireGuard, DMVPN and etc.) so in that case, you will not have any problems running the VPN of your choice.