Hi Sjordet,
I'm more familiar with Spectralink phone and had worked on Spectralink certification of HP APs, on of the checkbox you might want to try is the "Spectralink VIEW" checkbox under the Radio configuration, this checkbox will ensure a better care for frame tagged as voice.
The Spectralink voice certification is difficult to get, however HP had such certification, it does mean one cannot enable the "Spectralink VIEW" when using other vendor VoIP phones if those phones used is also SIP based like Spectralink.
The second thing you can do is to create a VSC dedicated for Voice, under Quality of Service, you force all the traffic for that VSC to "VSC Based Very High", this will ensure all traffic for this VSC will be processed as WMM AC=Voice.
You can also use Diffserv, however you need to know the DSCP code used by the phone and PBX to map them into wifi WMM AC=Voice, this is a little bit tricky unless you have good doc from the PBX and your phone telling you what DSCP code they use to send voice traffic, you can look at a wireless trace or wire to see this value.
But try my first suggestion to dedicated a VSC to "VSC Based Very High".
You can have another VSC for regular data traffic, however I don't recommend that since the VSC share the same radio, best effort traffic can block voice shortly because best effort frames have higher retries count than voice. You have to chose here, protecting voice or not.
Another aspect mostly unknown is most VoIP phone are using WMM-PS (power save) a.k.a. the famous UAPSD, this method of doing power save help the phone to be more responsive and yet still do some good power saving. Most respectable VoIP phones implement this features and most enterprise APs like HP, Cisco, Aruba have such support, the thing is both AP and client device have to have UAPSD, you can see this feature from a wireless trace when the phone try to connect to the AP and the AP advertises this capability in the beacon.
The third thing I want to to try is to increase the "scan ratio" by changing it from 0.5% to lowest 0.1%, this setting is under the radio config, the AP since 6.x release does a periodic scan for RRM and IDS purpose, the scan might interfere the on going voice call if the AP fails to detect a VoIP traffic, usually if VoIP is detected, the AP is supposed to postpone the scan until the voice call ends.
Please try the 3 suggestions I made to see if this can improve your voice call.
Another question I want to ask before ending this long message 8-) Are you under ETSI regulation domain? if you don't know you can just tell me the Country you configured the MSM controller.
Cheers.
