Yeah sounds like it is staying constantly connected via your data connection. Check it's connection settings or you could use something like Juice Defender and set the per app policy and/or also manage when and why your phone uses your data connection.
Voip use your data connection to make calls and not your cellular connection so it uses battery now as cellular service never quits you are still using the data connection to host the bandwidth of those calls when in call and a lesser connection when idle to poll for incoming calls as well (juice defender offers tablet mode to kill cellular connection and slow only wifi if you have no need to make calls via normal doodler or you don't mind toggling it to save battery). Also, depends on the developer and how they code their app as to exactly how all that is controlled, so different apps may cause different amounts of data usage to accomplish the same thing. You may find another app you like that allows you to manually enter another compatible services account.
Google voice is different it initiates a call to a call center phone line they item or operate as local to you as possible then dials your intended contact via that proxy call automatically for you and spoofs your accounts caller-id so it appears as your virtual Google voice number and not the proxy call center number it used to initiate the call. Thus all Google voice calls carry on the cellular side and don't require data.
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