[RFC v2 5/7] STE-plugin: Voice-call driver.

Updates after review: Use at_util_call_compare. So far, we have only done the trivial changes to voice call driver. We agree on moving more of the state logic to core, but this is complex and we need more time to discuss and look into this, particularly regarding call release. ---

Re: [RFC v2 5/7] STE-plugin: Voice-call driver.

Hi Sjur, Updates after review: Use at_util_call_compare. So far, we have only done the trivial changes to voice call driver. We agree on moving more of the state logic to core, but this is complex and we need more time to discuss and look into this, particularly regarding call release.