Why would you text back to a number that is unknown to you? You basically confirmed to whoever it was that your number is a valid number. People wonder why they get spam calls, well, that's why. Even if it was a robot call, they now know that they can spam call you. I am only giving advice on this. They constantly change their number, and they will call from a different number next time. Take it from someone who gets these spam calls all the time. I am only speaking from experience.
Received a call from this number, but wasn't able to pick up the call, so I replied a text saying "sorry I can't take the call right now", but got a text back said this is not an invalid number.
Why would you text back to a number that is unknown to you? You basically confirmed to whoever it was that your number is a valid number. People wonder why they get spam calls, well, that's why. Even if it was a robot call, they now know that they can spam call you. I am only giving advice on this. They constantly change their number, and they will call from a different number next time. Take it from someone who gets these spam calls all the time. I am only speaking from experience.