Here are a few resources to get you started on prioritising VOIP traffic on your router/network:
(1)VoIP Problems and How to Correct Them:
Video: https://www.youtube.com/watch?v=mnX6Im8GlJw
Presentation: https://mum.mikrotik.com//presentations/US16/presentation_3004_1462512668.pdf
Source: https://mum.mikrotik.com/2016/US/agenda/ID#0DMDMzO0A9
(2) Using RouterOS to QoS your network:
http://forum.mikrotik.com/t/using-routeros-to-qos-your-network-2020-edition/66683/136
You may also consider hirring a Mikrotik consultant to sort it out quickly.