External Extension Delay

I have a problem with my external extention. This is my setup, I have 2 cisco phone (7960 and 7912) in my house. they work fine in any ways, internal and external call. I also put a phone at my dad’s home (cisco 7912). When I call him from my home by dial his extension everything works fine. if I call him using my phone number and after dial his extension, everything works fine too. The problem is everything that coming from him .If my dad dial my extension internaly, my phone ring I pick up and it takes like 10 to 20 sec before we heard each other. after that it works fine. if he use his phone to call me on my cellular, for example same problem, it takes 10-20 sec before we heard each other

someone can help me