Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect that after that much of efforts and evaluation, the problem may be at your friend or correspondent's end of things. With any telecommunicative transaction problem, that can often turn out to be the case, provided the difficulty is only with one correspondent or distant machine you are communicating with.
It is, however, possible that you have a problem with an adventitious malware or virus on your device. If it is either an Android or an iOS/Apple device, they both have been found to have security holes in their operating software, through which exploits and viruses can enter the software workings of either the Android or the iOS systems on the handheld device.
If it is an Android device, then you can assess it by this testing procedure:
If your phone has (at a guess) the Android OS on it, then if that OS version is not sufficiently updated, then it may contain a significant 'security hole' that allows almost anyone to gain access and control over your phone.
To test as to that security hole, download and use this app:
to tell whether there is such a security hole in your phone's Android operating system.
To avoid the problem in the first place, not just using the Stagefright detector, but also completely updating your Android OS ab initio when you first start using it, is a good idea.
(Some older versions of Android OS cannot be patched, by merely updating that version... but a higher version, if it can run on the older Android OS -compatible phone, can cure that problem.)