A solution that worked for me on 4s, I was able to test and prove this between my iPad and iphone.
Settings, sounds, turn Vibrate off (not the one for silent calls)
Exit menu and turn the phone off.
Turn phone back on.
Settings, sounds, turn Vibrate back on, exit menu.
Test from another phone, all should be ok now, was for me.
Hope this works for you too.
I tested this for half an hour, the only way to stop the double tone was by turning off vibrate, turning back on without rebooting the phone just returned the problem, after rebooting the phone and turning vibrate back on all is ok.