echo during phone calls

hi

a couple days ago, I changed the powerbutton of a friends phone.

Given that I had not much time, I just took another frame, where the button was already working, and put the mainboard and speaker from the old phone into the new one.

He just called me and told me, that the button is working, but that is facing a strange echo while phone calls..

The assembling and reassambling worked fine so I don't really know where the problem could be...

Any suggestions? or did you ever hear about this problem?

could the cause be a dirty background-microphon near the headphone jack??

thank you in advance