as the vlans are in the same building, make sure the vlans can access the 3cx server network,
As PnP does not transmit across vlans, you will have to use DHCP option 66.
Another option create a vlan just for the 3cx network, and have the other vlan access it if you are using 3cx apps, etc. Depending on the switches you may be able to setup the switch to device voice devices and put them into the correct vlan. As the phones and 3cx server will be on the same vlan, PnP will work
No need for SBC,