I think many country needs their country if you need to register their telephone service(including VoIP service)
Taiwan VoIP service use 070 prefix...Mainly CHT chiand FET seednet...
But not sure having apps or SIP settings to use......
Japan have many VoIP service...mainly use 050 prefix...
050plus(provide by NTT)have apps to use it...
or fusion communications(skypein service seems provide by fusion)
if you need japan call-in number...you can try to register a skypein number...
Remember if you can register a number...you also need to pay calling free(always by minute)
If you just using in Hong Kong and calling to TW/JP fixedline number...you can try HGC0088(unlimited)
Or you can try nonoh...
Sometimes IDD maybe cheaper than TW/JP localcall charge...
eg:050plus local fixedline call JPY8.4/3mins...HGC0088 unlimited($56/month fee)...