Move the phone as far away as possible. Everything that I've read seems to pinpoint your suspicion, that being the phone. If you have done everything said thus far, then you have to keep the phone as far away as possible.
It's strange that the phone is far away yet still causing interference and apparently this is possible with LCD monitors. Something may be amplifying the signal as well explaining why it's so far away.
Mess around with the refresh rate. Typically I wouldn't recommend that but it's worth a try. Adjust it differently and see if this helps at all.
if this happens EXACTLY when the phone receives calls or texts I doubt it's the monitor cables which could also cause similar if not the exact same issues.