Can someone please explain why I may have a problem understanding my daughter on the phone.

It probably is, how she holds / where she holds the phone. Some people mash the phone into the cheek or chin blocking the microphone. Also some people insist on using speaker phone which can cause distortions.

I would guess she’s covering the speaker in some way. Next time you see her, maybe you guys could check it out? I know that if I hold my phone with my right hand, but am listening with my left ear, I have a tendency to place my thumb over the microphone.

Do you tell her you can’t understand her well and give her the opportunity to adjust the phone?

@JLeslie Yes. Actually I found my phone has a mono option and that seems to help a great deal.
I started to look it ups after I posted this question and it seems that the enhanced audio the newer phones have are either picking up to many sounds and mashing them but that it seems to have the most difficulty picking up the different frequencies or (I suppose they mean higher pitch) sound of a young woman.
I changed it to mono and I found that I am better able to understand her. Even when she has it set to speaker.
The same for when calling my husband in the car and he puts me on speaker to drive hands free. He was also able to hear me better and I was able to hear him better. You can still hear the sounds in the car between words but it completely cuts off the surrounding sounds of the car as I speak to him or he speaks to me. I think the new phones try to enhance all the sounds and you end up with a garble mess. Especially when people put it on speaker phone.

Very possible that she is holding the phone too close to her mouth and/or speaking directly into it instead of speaking over it or past it. I hold my phone so than the microphone part is resting on my chin.

The striking thing about your narrative is the fact that the difficulty understanding your daughter’s speech is restricted exclusively to you and your siblings. Since the problem is exclusive to your family and only on the phone, I bet you could discover the cause through systematic elimination of Likely factors, beginning with the differences between your daughter’s voice & patterns of speech and your own. Was she reared in the same region of the country as you and your siblings? What characteristics in her speech are amplified or suppressed by the telephone. Perhaps you & your siblings share a common characteristic in your ears. Do things improve or deteriorate depending on the phones used? It’s an intriguing situation, & I bet there’s someone out there who’d love to do research on your fascinating anomaly.

My boyfriend has this problem with me, lol. Its the way I hold my phone sometimes though, I dont hold it directly to my face and I don’t even mean to do it/dont even realize it.

I think she figured it out, guys.

It wasn’t just holding the phone incorrectly it even happened when people were using speakers and hands free. Only difference with my daughter it was any which way she used the phone.
As @Dutchess_III pointed out. I did find out what the problem was and I am now able to hear and speak with my daughter and anyone who puts me on speaker.

I explained above what the problem was so that anyone, who has this same problem may be able to adjust their phones.

I have problems communicating on the phone also. Only about 7% of communication involves actual words. 55% of communication is visual (body language, eye contact) and 38% is vocal (pitch, speed, volume, tone of voice). Perhaps your daughter communicates with strong body language. Body language can reflects confidence, competence, and charisma.

She figured it out @gondwanalon.

If others can understand her then it’s your hearing! I have trouble understanding when someone speaks too fast when others can understand their speech well.

I think I’m going to scream. If anyone wants to post with a solution again, please read this, this, and this please.