That's not an old system. Mitel is very reliable but also complex. Getting a tech there is your priority at the moment. How are you operating if you can't make an outbound call? Especially in the case of a guest emergency! Get it repaired and now you have someone you can rely on for the SIP trunk setup. I recommend a SIP fxo gateway to supply analog trunks to your Mitel. That way it's just a swap of dial tones and you aren't investing in SIP trunk licenses or reprogramming the Mitel.