If I live in CT and my employer is based in MA, but I telework 100%, which state/s do I pay income tax too? Which state/s should I withhold income tax for? Thank you!

Will you be receiving a W-2 from your employer? 

You probably need to speak with your employer regarding state withholding.

Q.  If I live in CT and my employer is based in MA, but I telework 100%, which state/s do I pay income tax too?

A. CT

Q. Which state/s should I withhold income tax for? 

A.  CT, if you employer is willing to do that.  Some employers are unwilling to do that, in which case you will need to make estimated quarterly tax payment to CT.  Ask your employer not to withhold MA tax; they are not required to do so, in your circumstance. Reference: https://www.mass.gov/guides/withholding-taxes-on-wages#:~:text=If%20you're%20an%20employer,filing%20and%20paying%20withholding%20taxes.

A few states (New York,  Pennsylvania, Nebraska, Delaware and Arkansas ) tax telecommuters, but Massachusetts is not one of them.