Unable to facetime showing "choose another phone number or email address to call", Tried all the possible fixes that I can find from internet.

I have been experiencing trouble in facetime in my new iPad to facetime with my dad.

Few facts at my dad's iPhone 15 pro max

1. My dad is using an IPHONE with latest OS, software updates
2. My dad already activated/enabled facetime in settings. he already set phone#, email, icloud email to be used to contact for facetime.

Few facts and tests done on my iPad pro 2024 Wifi only

1. I added his phone#, email, iCloud email into the contact in my iPad. We can iMessage without problem.
2. "facetime audio/video call" button is not showing up in his contact at all.
3. When I attempt to call my dad, it pops up: "choose another phone number or email address to call"
4. My dad invited me via a link, I opened the link in a web browser on iPad, facetime worked successfully!
5. My dad invited me via a link, I opened the link in a web browser on Android phone, facetime worked successfully!
6. My dad invited me via a link, I opened via facetime App, I can join, he can join as well. But we cannot see if each other has joined. Both of our devices were showing "waiting for others".
7. I have full software update done to iPadOS 18.1
8. I did network reset, full reset, full factory reset. None of this help.
9. I flipped the toggle of enabling and disabling facetime in app settings.
10. I deleted my dad's contact and added the contact as new.
11. I deleted the facetime app and reinstalled via appstore.
12. I did multiple system restarts
13. I called Apple support and they cannot fix this. Saying "ok again apple is aware of this". I mean. what the..

Now nothing works and I don't know what to do.

I have other contacts that works perfectly fine with facetime. hopefully someone can provide some advice. Thanks!