Here's a good one for all you cell phone technology geeks out there.
My mother has a Jitterbug. My dad has a T-Mobile smart phone.
When my mom calls my dad from her Jitterbug, his smart phone doesn't ring, it just lights up. (Unless it's in silent mode, then it vibrates.)
When my dad calls my mom from his smart phone, her Jitterbug doesn't ring, it just lights up.
When each of them call other people, no such problems.
When each of them receive calls, no such problems.