Why am I getting SMS from our other phones on the phone I forwarded them from

OK, we have 3 phones all on T-mobile linked to the same account (Magenta 55+). One phone replaced an AT&T landline. The other two replaced AT&T mobile phones. (Sense a theme?) The landline replacement gets the doctor calls, neighbors, friends, relatives, so people only have to know one number. The others are for personal calls from people needing to reach us individually. I want to forward SMS from the landline replacement to the other two phones because the landline replacement is rarely looked at.

I did that successfully with Tasker. But when a message is forwarded, it comes back from the other phones like they forwarded it to the landline replacement. Tasker is not on those phones, so I don't see any way it can cause this.

Those phones are all linked on the same account because we got them all at the same time, and I am the only one who has half a clue how to access info online, including DIGITS. My wife's phone is not registered to DIGITS because I don't want to overhear communications with her lovers or whoever (as if! Not that I would be disinterested if I were single!). Just a privacy & respect thing.

Is there some T-mobile setting that causes the other two phones to share SMS messages back with the landline replacement that forwarded them in the first place? What exactly does linking phones or lines in the my.t-mobile account dashboard do? Could it cause this? The landline phone does not get any other SMS messages from the other phones. Only the ones it forwarded, which clearly come back from those specific phones.