but can receive calls. Why ?

I'll try and get on our POS phone service chat tomorrow and set an appointment,  hopefully they fix it before.

But curious what would cause this

Tip and ring got switched somewhere along the line.

eta-If you have access to the terminal block, swap the polarity on the two wires.

take the phone outside to the phone junction box on the side of the house. unscrew the one screw available, open the box. There will be a phone jack in there. Plug your phone in and see if it works. If it does, the issue is somewher inside, possibly wiring.

I keep an older corded phone for emergency use, you can use one on any junction block(or could)

Believe its system issue,,getting  "carrier is busy"

I'm 99.9% certain, sure you'd pick up on it or realize of you did something inside. So Very seriously doubt its that.

If you have tried a second traditional land line (POTs) phone both inside at an RJ-11 Jack and outside at the Telco's Demarcation enclosure's RJ-11 Jack, I'd guess someone (not sure why) has changed something either at the local PED/box in the field, the Central office or your accounts provisioning.

Keep us updated, I'm curious what the root cause and the Fix was.

Who is your Landline Provider?
Are you in a Rural area or Metro?

I'm curious if they'll even fix a POTs line or just switch you to a Cell based VOIP box that give you landline Tone and type service.
Its no secret that many Telcos are allowing copper to die.

My pops shut off his ATT phone in St. Louis, because he is a bit of a cranky old Luddite sometimes.   Anything he doesn't like (can't understand, fix or make do what he wants), his solution is to "Just Turn the MF'er off and Cancel it"...
He did that one day in a tantrum.   Two weeks later when he could not call his brother to bitch about the clouds, he discovered the error of his ways when he called ATT to turn it back on.  They said, Sorry, we are no longer turning up or supporting Copper lines in your area.  We will switch you to a VOIP setup.  They put a Cell based MTA in his house and plugged it into one of the Pots jacks and brought all their landlines back on. Now with a new a new phone number as well...

This is a very likely cause and fix.