Snom 300 one way audio to Mitel 3300

Hi ,anyone that can help point me in a direction ,i have a TB connected to Mitel 3300 using SIP trunk ,i have SNOM 300 phone connected to TB ,i can phone a PSTN number across the Mitel trunk and the audio is fine ,but if i phone a extension on the Mitel side i get one-way audio. eg. i can hear the person talking on the SNOM phone ,but the person on the Mitel phone can`t hear me talking.
Any ideas ,,please