One-way speech across SIP trunk

Hi guys
I have a Asterisk system connected via a SIP trunk to a Mitel 3300 PBX ,the connection to the PSTN is on the Mitel side ,i can phone out to the PSTN from the Asterisk box without any issues ,but when i call an ext on the Mitel side from the Asterisk side i only get one-way speech ,eg i can hear the person on the Asterisk side but the person on the Mitel side can`t hear me.
Any help will be greatly appreciated.

If you can call out thru the Mitel system to an external PSTN okay, but not to a Mitel SIP extension i would suggest that the problem resides on your Mitel system.

Disable reinvite on asterisk and see what happens… Are they on the same LAN? no firewall between them?