Currently VoXalot does not support CallThru....
However, it can be achieved if you are willing to do some work
(btw, instead of configuring multiple trunks, simply add VoXalot as a trunk for outgoing calls, since dial plans in VoXalot are much easier to work out):
Placing VoIP Calls from any Old Regular PSTN Phone
If you also want to receive calls, a VoXalot Premium acocunt will be needed then....
Please read the guide and tutorials here, and make sure you understand what you need to do, before going ahead with anything... (the guide is not related with VoXalot and VoXalot cannot provide support for the process)