Cannot facetime and imessage with one person/contact

i use the same device to facetime & imessage my father since few years ago till now. Recently, while i'm facetime him, his ipad isn't showing any incoming call. Then if he is calling me, i can see that incoming call, but after i click on green phone icon to pick that up, then it shows fail, and it alerts me with a missed call. With imessage, after i sent out a message, it showed a blue bobble without any error message; however, he isn't receiving any message from me. The weird things is, I can receive his imessages...

my father's ipad doesn't have sim, so I only have his icloud adress as contact info.

with the same device, i can facetime and imessage with other contacts on my phone, also my father can do the same with all other contacts on his ipad, but just me.

I already searched for answer from other threads here, but seems no solution.

Anyone is having the same issue and know how to fix that ? please kindly help here.

Thanks in advance.