Unless your Mitel has a PRI card and Q-sig option enabled and link license, the only way to tie the 2 systems together would be T1 E&M Tie trunks. The card you listed does CAS. It appears that you have 3 T-1 circuits if fully populated would mean 72 trunks. Don't know if 1 PRI will be enough. Yes you will need a T-1 crossover.
