Troubleshooting call quality degradation for VoIP phones when using handset versus speaker

Phones are a combination of Polycom VVX150 and VVX 201. When using the speaker, calls are perfect, when using the handset, the voice quality degrades due to something network related. Location is using VLANS, has a Velocloud router, and uses Zscaler for a proxy.

Does anyone know what packet header information is related to how the calls are treated internal to the phone? Apparently, at least with the Polycom phones, packet headers do contain information about whether they are to/from the speaker or the handset within a VoIP phone and something on the network is treating the handset packets different than the speaker packets.

Thanks in advance for any suggestions.

There is info in the call set-up packets that tell a receiving device whether or not to go to speaker. Once the call is set up, though, it shouldn't matter how the audio is being rendered. It might help to know if it happens at both ends of a call, if it's only on incoming calls, if it's between similar or different devices or if it happens only on internal calls or if outside calls act this way.

Make sure the phones are the latest Version software. The 201s are old phones. Upgrade them