You can't trust that the endpoint of a call is on a secure network. If you need to interact with multiple stakeholders from an organizational POV, you may not trust identity of the other parties either, so a traditional Skype/Teams/Jabber type thing won't get it. Signal lets you do that ad-hoc, at least for low trust level validation.
The VPN provides security for the LTE or Wifi connection. The network that the VPN terminates on may not be a "trusted" network either. Think of an office LAN. You usually don't require a VPN to join the office Wifi or wired ethernet. The LAN provides a level of trust sufficient to access the printer or something similar, but you still secure a connection to your server with SSH.
The VPN provides security for the LTE or Wifi connection. The network that the VPN terminates on may not be a "trusted" network either. Think of an office LAN. You usually don't require a VPN to join the office Wifi or wired ethernet. The LAN provides a level of trust sufficient to access the printer or something similar, but you still secure a connection to your server with SSH.