You can do it by modifying the value of  status_msgs_with_audio_indication removing the DateOngoing and DateReminding status messages.


status_msgs_with_audio_indication=PhoneHasIncomingPublicAnnouncement DateOngoing/3 DateReminding/3 CallBackOnBusyAvailable/2"

should be changed to:

status_msgs_with_audio_indication=PhoneHasIncomingPublicAnnouncement CallBackOnBusyAvailable/2

The setting  status_msgs_with_audio_indication isn't managed via web interface, so can be configured via provisioning or via HTTP request.

Otherwise you can deploy it via inband provisioning using a custom client policy:

To push this to a large number of phones simultaneously using Lync Native Device Management you will need to modify the existing Client Policy for the phones and use the cmdlet New-CsClientPolicyEntry with Lync PowerShell.

For Example, if the Client Policy were named 'snomPhones' 

$x = New-CsClientPolicyEntry -Name "snom_status_msgs_with_audio_indication" -Value "PhoneHasIncomingPublicAnnouncement CallBackOnBusyAvailable/2"
$y = Get-CsClientPolicy -Identity snomPhones
Set-CsClientPolicy -Instance $y

That should disable the EWS message notifications on the snom phones