Ringtone / speaker during call does not have sound if Spotify is playing a song before the call (iOS)

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Reproduction steps
    - Open iDoubs
    - Resume/Start playing a song in Spotify (by using the pull-up quick menu)
    - Receive a call when iDoubs is in foreground
    - Pick up the call

Expected behaviour
Ringtone has sound. Speaker also plays out the sound from the caller after the 
call is picked up.
Actual behaviour
Either ringtone or the speaker has no sound.

NB
So far Spotify is the only application we have found that does not work 
alongside Divide / iDoubs. However, this issue is not reproducible using Skype 
and Spotify.

Original issue reported on code.google.com by boss...@yahoo.fr on 5 May 2014 at 7:18