improving communication on site

Hi folks,
We are running 256kbps Optus shiron services and trialling a Call Direct CDM-790 NextG Box connected to 14dbi Yagi(Vertical). So far the reception is inconsistent works well at night but not during the day when needed the most.
4 Voice lines are provided by an NEC DMR that feeds backinto the PABX at our Melbourne office, a Beam RST-100 provides redundancy via Telstra iridium satellite. Latency can be issue for the voice but is generally quite good, most complaints around slow network access. A juniper WX-20 WAN accelerator is on site but is primarily used for QoS.

Could anyone provide me on how we can improve the current setup or what we should be using instead??