When I have left the country previously, I contacted China Mobile prior to my departure and told them I was and that I'd like my phone to still work. they said, OK but that I had to add money to my account before I go. 5000 RMB as I recall, but that was a few years ago. Once the money was in my mobile account, they unlocked the phone OTA and I was able to use it abroad. Roaming charges are high, so I used it not much. But I did want people here in China to be able to reach me by my China number. Messages came through and twice people tried to call me on my China number. the calls came through, but I told them quickly that I was not in China at the mo and to contact me by email with whatever they wanted. They understood without further explanation.
Don't know the answer, but I can wager a some guesses: (1) your account ran out of money; (2) you hadn't set up mobile roaming through CM and when they started getting calls/SMS trying to connect to their system from abroad, they locked your account-kind of like how western banks will lock your credit card if its trying to be used from outside your home country; (3) both of the above.
If you're going to be traveling outside the mothership, and plan on trying to use your mobile and/or Chinese bank credit/ATM card, contact both your mobile carrier and Chinese bank(s) prior to climbing aboard the exit vehicle.
Also don't forget to notify your FAO before you leave and after you return so your Residence Permit status stays updated in the PSB. The PSB will know when you have left, but by their rule, they have to be notified that you have returned. That's your FAOs job.