Don't think it's possible on SIP stations, doesn't show up as option in programming. And even if you could, I think fax over SIP is not supported at all on 5000 so it might be pretty unreliable. I assume you're running IP phones (if digital, there's actually a digital to analog adapter you can still find on secondary market, called Single Line adapter). If all the module bays on main chassis are full, nowhere for another Single Line Module-4, you could add a DEI chassis and more SLM-8 there.