T-mobile wifi calling connection problems with Airport base station

My family and I have purchased phones and service with T-mobile hoping to use their wi-fi calling at home to reduce the minutes we use on our plan (wif-fi calling is free) The phones we have are the Samsung Katalyst and the Nokia e73 mode.

The Katalyst connects with no trouble to our wi-fi ( wi-fi runs through our older airport base station and newer airport express depending on where you are in our big house) but half the time calls on our end are horrible quality, its cuts out about every other syllable.

The e73 can connect to the wi-fi only for web and app use not for wi-fi calling. When I click to connect to the network it comes up with "W006.1 ISP error. Retry" The same thing happened on another wi-fi network but not on one other.

I don't know if I have a router problem, a phone problem or a t-mobile problem. The guy at the t-mobile store didn't know how to solve either problem, though he did try for a long time.

In my looking online I see that some people had the W0006.1 error message and fixed it by setting their router to "allow fragmented packets". I'm not sure if this is something that relates to an airport base station. I have found nothing in the settings to change anything to do with packets (except one option to allow larger packets but it was grayed out and didn't allow me to select it)

I am not super techy so if you have some techy advice you are going to have to dumb it down for me, sorry.

Any help or suggestions would be greatly appreciated!