Even in a isekai doing videocalls with only the upper clothes is a common technique.
It seems like the call is at the same time of MC smashing the teacher, so maybe the other MILF was self-pleasuring when it ringed?
Now, someone will ask why MC didn't put things straight with MILF teacher, simple, when a man is given a meal he can't refuse it, could have explained things later in sage mode, but had to straighten her back before (all that booba must be heavy).