I'm not sure VoIP calling on mobile phones and tablets is strong enough yet to count on it replacing a cell voice line. Call quality isn't always as good (depends on your hotspot and WiFi signal), and presence isn't as reliable as a phone's (sometimes the client goes offline or away when you don't expect it).
Gtalk for voice and video calling works on 10.1" Android Honeycomb tablets, but for Android OS Froyo and Gingerbread (except the Nexus S right now), it's IM only. Skype is a better bet, or other VoIP apps like Fring (though I'm not in love with Fring). Skype does provide calling to landline numbers but you have to pay, only Skype-to-Skype is free. Your potential money savings might go down the drain.
The HP TouchPad actually has a very robust Skype implementation, but I don't know if you want to use a 9.7" tablet as your phone.
As for the best hotspot, much of that depends on what kind of coverage and services are available in your area. Verizon's LTE simply rocks, but only if you have LTE in your area. T-Mobile's HSPA+ 4G is very strong in my area, but their coverage might not be good in yours.
Another option might be a T-Mobile 4G phone with wifi calling (a built-in feature). That doesn't use your plan minutes but you'll use your existing cell phone number, which is convenient when others call you. Recent phones like the HTC Sensation 4G and MyTouch Slide 4G have this feature. You could go with a data plan (required anyway) and a very low monthly minute plan and use WiFi for calling whenever possible.
-------------------- Lisa Gade Editor in Chief, MobileTechReview