If you are behind an “only necessary services are permitted; everything else is blocked” firewall, you may be out of luck. If your Pi is new enough to have Wi-Fi, you could set up your mobile phone as a hotspot and connect the Pi to that, avoiding the corporate firewall.
Assuming that you are in US or Canada (if not, please post your country), you might try:
SignalWire; they give you a small credit at signup so you can test without making a payment. This is standard SIP, so it won’t work if your firewall blocks it.
Callcentric; also SIP, but uses nonstandard ports so less likely to be blocked. A free account can make toll-free calls and receive calls via SIPBroker. If you get past that, you can deposit as little as $5 to make and receive regular calls.
VoIP.ms; offers IAX connectivity, which might bypass attempts to block SIP. $25 minimum deposit but unused balance refunded if you close the account.
CallWithUs; offers a VPN through which you can do standard SIP.
Another possibility is setting up your PBX on a cloud server, where it will have unrestricted connectivity. If you can’t find a way to get your extensions through the company firewall, you could just use external numbers for your demo (calling an extension would ring to a corresponding mobile phone).