There’s a property of each bridge port called hw. I guess that if all ports on some bridge would have hw=no, then that bridge then wouldn’t be picked for HW offload.
If one executes (in CLI) command /interface/bridge/port/print, only ports of HW-offloaded bridge will have ‘H’ flag shown in flags column (between index number and interface name).
Yes. You could set ether1 (ISP-facing port) with PVID X, ether5 with same PVID X and this would establish ethernet connectivity between IP phone and ISP.
IF the phone doesn’t use VLANs (which they often do). If it does, then you have to find out the VLAN ID which your ISP uses for VoIP and then configure both mentioned ports for hybrid use (tagged VoIP VLAN and untagged with VID X for internet).
Then you could set the rest of ports with PVID Y.
And then complete config with bridge port, bridge interface and vlan interface config (to unwind your brain off these bridge personalities check bridge mysteries explained … it actually helps to understand tutorial linked in next paragraph).
If you’ll look into VLANs, then study this excelent tutorial. None of examples directly apply to your use case, but you should get some insight.