Collection Agency not responding back to me via mail

I received a letter in the mail from a collection agency on Jan 21 about a debt that is owed. A counter offer was sent via certified mail but have not gotten a response from the agency. I did put in a date of March 8th to be the deadline to be able to send the money in form of a cashiers check or money order if they respond back with an agreement. Also the letter they sent me does look like a validation letter having the: name of the creditor and the amount presumably owed. Also the offer was made to pay only half of the amount they said was owed. I’m also not seeing anything yet being reported on my credit report about my debt being sent to collections too which I find odd. Should I keep sending my offer via certified mail once every 30 days to see if I get a response? I don’t feel comfortable calling them either.

Update: I did some reading, and I didn’t realize that I would have to pay taxes if I settled for half of the debt. I will probably just set up a payment plan option to pay all of the debt that is owed so I don’t have to pay taxes.