For the OP In the Hospitality industry, you can do a couple of things, but first, REPAIR the PBX. I'd be looking for a second opinion, because, if indeed you are getting calls in, but not out, it might be something silly like PROGRAMMING something simple like loop start vs. ground start. Then you can get half-a-loaf by converting your trunks to SIP. An 8 or 16 port gateway in front of the Mitel trunks with Sip service will pay for itself in reduced trunk charges in a mater of months. The last one I did the payback was less than 6 months for the hardware and monthly difference in rates.

Carl