Why can't I change / alter / modify my landline service online?

The why you'd have to ask AT&T, we as fellow customers have no idea why they do a lot of the things they do. 

That would be because landlines will soon be obsolete, therefore consolidating the amount of assistance and support they offer. Recommendation: if it’s not a business landline, get rid of it, upgrade it to a smartphone if you need that number.

As if I can get through to a human...

I know there are AT&T employees in this forum, so maybe they can enlighten us.

They are going to tell you to contact customer service, it's a catch 22. Have you tried chat? I don't know if it's any better or not. 

@MildlyBothered -- There are 2 problems with that solution:

One is my alarm company requires a wired phone connection.

Two is we lose internet and power ALL THE TIME in my area, and when that happens a digital phone is 100% useless. The only true reliable phone is a wired landline that does not require internet OR electricity to make a call.

Then that explains why you would need that 😭 have you tried going in store (corporate) to see if they could assist instead? A little bit better than being on hold for an hour in my opinion :)

I suppose I could do that, but the last time I tried, all the store lackeys tried to do was sell me new cell phone service (which I already have) and none of them had a clue about landlines.

Power outages can be somewhat overcome by battery backup. You'd have the issue if you used a cordless phone system. I use UPS's on phone and computer, anything over a couple hours I break out the generator. 

@spoom2 - I TRIED the chatbot and it didn't even know the word "landline".

I finally got through to a customer service agent, and after 15 minutes of "wait, let me see what your options are" he told me they only deal with wireless accounts and I'd have to call the @#!$ 800- number, which was what I was trying to avoid in the first place. 

On a side note, I have a lovely landline telephone that is from sometime in the 1970's. It weighs about 9 pounds and can be used as a bludgeon.