I'm confused

I've written to my penpal for quite sometime and so far so good. We both message each other everyday and have interesting conversations. His english is also getting a lot better.
Since it's been awhile and we are more open to each other i kind of want to start voice calling, although I'm not sure how to bring the question up.
Is there anyway to make it subtle and not as awkward...like a hint??

One of my penpals "accidentally" called me once (through LINE). I wasn't around to notice the call, but when I did, I was greeted with urgent messages of apology. She probably really did hit the call button on accident - but, it did bring up the idea of calling instead of talking through text. So, there's one way to ease into it. Maybe call "accidentally" and then that will give you a chance to see how your pal feels about voice chat.

Just say that you want to do the voice calling if he has a voice equipment set.

Of course, being upfront and honest about your intentions is another solid way to go about it. But, some people have anxiety about doing voice or video calls, and once you ask them directly, they may start avoiding you or feel uncomfortable when talking to you if they know you expect voice chat. Instead of disappointing you by just directly saying "no, I'm not comfortable yet", they may just slip away.

Of course, that example does not apply to everyone! Some people are more open than others, no matter where they come from. I've just noticed that my japanese friends seem more indirect than I expected.

I suggest you not to push him for video calling. Some people feel uncomfortable with video calling. For the most part, it has nothing to do with them being weirdos. It's most likely self consciousness.
I have also suggested to my Japanese pen-pal on numerous occasions to try video calling, but he refuses every time. I just wanted to see him in motion picture, that's it. But, I understand that he doesn't feel comfortable doing this and frankly speaking, neither am I. We are both very non-photogenic (we don't look good in photos at all) and I guess this is true for the webcam as well. No worries though. Maybe if you meet him in person, things will be different and I hope you guys enjoy being together. Good luck!

Thanks for your answers and ideas guys!
I agree, it might be awkward to do a video chat or a voice chat. I will definitely respect my friend ^.^
Maybe in the future I'll bring up the question,but for now I'd want to stay friends. I can't risk my friendship...haha
The only positive thing i could look forward to is that he wants to show me around when I study abroad in 2016~
Thanks everyone

Japanese have learned English reading for years, but not listening and speaking. that is a reason. you will be surprised that English teachers of Japanese in public schools do not speak English fluently.