Slightly off-topic perhaps. But I have been using the same Chunghwa Telecom pay-as-go account for 10 years or so. I haven't bothered with data (conflicting stories as to whether I can get it anyway). Just use voice. Slightly annoying it won't roam. (I'm told that no PAYG SIM roams, but again conflicting stories.)

Anyway I usually stick it in my phone on arrival and away I go. But on arrival at TPE a few days ago it failed registration. The SIM was working fine, but the network wouldn't take it. Two or three reboots over about an hour and bingo it worked. Could be a faulty SIM card, but if so I wouldn't have expected it to work at all.

Aside from that, this account has served me well over the years. I always keep a spare top-up card so if I should ever be out of Taiwan for 6 months or more, I can top-up remotely to keep it going. Hasn't happened so far!