We currently have a DrayTek Vigor 2866 which due to location purposes cant take DSL due to mega low speeds so we are using a TP-LINK 4G router to feed into the WAN2 of the Draytek.

We have a bank of VOIP phones (hosted with Daisy) and they connect fine.
You can make external calls no problem 100% of the time.
When you make an internal call from one phone to another *SOMETIMES* the audio doesnt connect the metrics of making the call and timing the call all work fine except audio.

Any thoughts?

Our initial thoughts were the 4G however, we think there might be some double NAT going on from the 4G router and the Draytek causing this.
Anyone had any issues similar?
One floating idea was to make the 4G router a DMZ to the Draytek but want some other thoughts if anyone else encountered a similar setup

NOTE: All phones are located inside one building all on the same network and VLAN