Delay at start of SIP call

I’m running Asterisk 11.7 on Ubuntu 14.04 with FreePBX 2.11 with several SIP trunks and using Android 4.4 phones with the inbuilt SIP client as extensions. When making calls there is a delay of a few seconds during which the person on the other end can’t hear us when first answering the call. We can hear them without any issue. After the first few seconds it starts working fine and there are no issues for the rest of the call. There is also no such issue when receiving calls.

I’ve been looking at this issue for sometime but can’t find the cause. Does anyone have any pointers as to where to look or what the cause might possibly be?

Thanks, Dave.