Check your trunk to trunk allow tables to ensure the Main Groups of the various trunk groups are allowed to send traffic between them.
It's easily forgotten.

LTTA


Worked examples

This example shows allowed connections for trunk main group 14.

IN OUT IN OUT Allowed connections
001 014 a) Bothway: within group 14
014 002 002 014 between groups 14 and 2
007 014 between groups 14 and 18
014 014 014 014
014 015 b) One-way: between groups 1 and 14
014 018 018 014 between groups 7 and 14
between groups 14 and 15

Revise with RTTA


How are you connecting the DX to Mitel?
SIP, QSIG, Q.931.

If SIP then do you have a number length table associated with the trunk?
SIP requires En-bloc dialing ,rather than overlap dialling. The number length table will do this

In the example below Main Group 34 has Number Length Table 6 associated

D A R D D D T M N R
MG TYPE CODE / OUT SRCH SEND PSD S T IN D T REG T S OG D L A PE
A T T T M T D
034 DPNS 104 D HRS FXD A DDI N N 00 N 06 N

In the example below digit strings beginning 2 will wait for 5 digits (2XXXX) before sending the digit string.
H8K is used for SIP, regardless if you have an Openscape 8000 for not.

S?lnlt 6 2
NUMBER SEIZE FLAGS
DIGITS LENGTH ACTION LENGTH
2 05 05 H8K

Might need to do some tracing to see what is happening.

Cheers - ended up being a BT issue in the Embark