While Bandwidth Manager doesn't support prioritisation, you may be able to handle all VoIP traffic in a separate rule with unlimited bandwidth.
You could try making a rule like this and placing it at the top of your ruleset (it has to be the first rule):
Source: IPv4 Range: 192.168.100.1 - 192.168.100.254
Protocol: IPv4 based
In the Add/Edit Rule window for the rule, go to Advanced tab, click "Even more advanced settings", and then tick VoIP in the DPI Matching section. The rule should now handle VoIP traffic, while everything else will be handled by subsequent rules.