In case you're using Webex Calling: You'd need a Common Area Addon/Worspace License for the board in order to assign a PSTN number to it. You cannot use the "higher rated" Basic/Enterprise or Professional/Enhanced licenses..
Yes, it is possible to tie an on-prem phone number to Webex Board registered in Webex Control hub. If you have on prem CUCM then you need to complete Hybrid Calling Deployment for Cisco Webex Devices using Device Connector. This requires Expressway C and E deployment. Please follow below deployment guide: