Default User experience with calling apps

Hi. I have a tmobile plan 100 minutes talk and 5GB talk. I've also added a tethering option to this plan. I was wondering, since my plan is for 100 minutes (10 cents a minute after 100 minutes is used up), that it might be good to have a back up plan where I use an app on my tab pro 8.4 for calls. (Guess I could do the same on phone but thought it might be more fun with the tab pro). Anyways, could folks comment on their experience in regards to what apps they like to use to place calls on their tab pro. Thanks