What is VoIP Phone service over the internet and do I need or want it?

This type of phone service grabbed my attention because I now have pretty reliable, stable, high-speed broadband in my rural area and I want to save money on phone calls.  I started researching what it was, how to get it and what to watch out for.

Although I now have AT&T Home Wireless for $27 (includes $7 Fees added to base price of $20 so that's about 30% already), those fees keep going up and although stable as it gets wireless power from a nearby cell tower, voice quality is somewhat muffled and so can I do better?  I don't want the local land-line--dropped them because fees were close to 30% of my bill, so around $56/month.

It is portable and battery operated--I can take the base unit with me along with a corded phone and get "home" wireless service anywhere I travel for that same $20 a month, as long as I can find an AT&T tower close by. Similar service by Verizon.

It connects to our Panasonic cordless base unit and feeds 5 cordless phones around the house.  We also have it feeding to a leftover land-line system that feeds phone outlets throughout the house for corded phone connections. It is NOT VoIP, but cellular based. BUT I CANNOT FAX with it.

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I started with WikiPedia help at https://en.wikipedia.org/wiki/VoIP_phone  

I see that I can get a similar base unit that hooks into my Wi-Fi router (that will, at its sleak download and upload, furnish the guts to run this type of phone) either by a wireless dongle or ether-net direct (probably the best choice).

I am about to try the Ooma at https://www.ooma.com/telo/home-phone-service/ 
from an Amazon special that if I don't like it, I can return in 30 days.  I can port over my old number, but won't do so till I know it's stable. I have read reviews which are mixed and seems the bad reviews are due to using wireless from router to the base unit OR having unstable internet. I CAN GET THIS SERVICE FOR $ 3 TO $4 A MONTH!!!!!!!!!!!!!!!!! 


It will include what I have now, Caller ID, Call Waiting but not sure about 3-Way calling or Call forwarding, Voicemail.  I think I would need to get the $10/month plan to include Call blocking, 2nd line, 3-way calling and call forwarding, so total of $13--which still saves me $14/MONTH (and pays for the equipment in about 4 months) with more features and flexibility. That plan ALSO includes free calls to Canada and Mexico!

BUT can I trust this phone service?  How likely is my internet to fail vs the Cell Tower?  With what I save, I can probably expand my cell service as a backup when needed, but my internet service will have a battery backup also and I have been told in past 4 years, maybe only 3 hours downtime for upgrades, so am hoping I can depend on this Internet-based phone service.  Will it be easier to hear? Will soon see.....

Will I be able to hook it up to our cordless phone base unit to feed the whole house and also take it on the road for "Travel" calling?

Stay tuned for what happens when I try this new cost-saving system, which is supposed to arrive tomorrow.