Assuming you actually need/want/require prioritisation you need to use either simple.qos/fq_codel or layer_cake.qos/cake and then you need to make sure your VoIP packets are appropriately DSCP marked so that they end up in the desire priority tier/tin. I would probably recommend to use layer_cake.qos/cake and maybe even switch from the default diffserv3 DSCP-to-priority mapping to diffserv4.
But honestly, that is the easy part. The real challenge is to make sure that VoIP packets are appropriately marked before they hit SQM and/or the APs...
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check what DSCPs these devices already use for packets they send towards the internet by recording a packet capture and look inside with e.g. wireshark.
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Make sure you assign static leases (OpenWrt GUI -> Network -> DHCP and DNS -> static leases) to all your VoIP devices, so you can easily find them in packet captures and use iptables rules to mark them.
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for incoming VoIP packets from the internet it is very unlikely that they arrive appropriately marked, so you will need tc filters or (maybe in a coming snapshot version of OpenWrt) the qosify application to selectively set the correct DSCPs for your policy.