VoXalot does not yet support this feature...
However if you follow
Placing VoIP Calls from any Old Regular PSTN Phone you do not require a Premium VoXalot acct. (although a Premium acct. has some neat perks in terms of incoming DIDs, VM on the WEB, and Calling Records)
That guide seems a bit overwhelming on first read, but following the steps carefully helps (feel free to ask questions here or via PM if you need any help with the guide)
Keep in mind that there are some limits:
1. Most Betamax services have a limit of 300 free minutes/week (and you probably already now about the FreeDays)
2. PBXes has lately made changes that allows you to use them for only up to 2000 min/month with their free acct. Because the way callthrough works (time counts for you calling in, and for the calling out) that translates into 1000 minutes that you can use PBXes for CallThru per month....