[voicecall] Fix compiler warnings

Fix 'unused variable' compiler warnings introduced by commit 6aafbe93744b366518fe5cd69661d5998ae70958.

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>
Merge pull request #141 from marttipiirainen/next

[voicecall] Fix compiler warnings