Modified Sine Wave plugin?

Steve made an Ultra-narrow filter plug-in which can be downloaded here … Trying to isolate sound in a certain frequency... - #11 by steve

If you make two copies of your “system” dial tone (with the unwanted frequencies) , then apply Ultra-narrow @ 350Hz 5 iterations to one copy, then apply Ultra-narrow 440Hz 5 iterations to the other copy, and play the two filtered copies together, you get a dial tone which is even purer than the “regular” one you posted, A before-after example is attached ….

The phase between the 350 and 440 may be responsible for the modulation effect you are hearing,
If you apply ultra narrow filter to the two copies as I have described above, you can alter the phase by shifting the two filtered copies out of sync by a few samples.