If you do, then get the SPA3102 which can be setup to do exactly what you want without using 3CX. You can go VoIP in, PSTN out. Security can be setup using caller ID or a PIN number to get dialtone from the second line. It behaves more like a true DISA port.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.