MTK stack unable to send MO-SMS on TCH/F
When a phone with MTK chipset (and firmware) wants to send a MO-SMS, it is unable to deliver that SMS if the BTS/BSC assign a TCH/F.
The MS will respond to the IMMEDIATE ASSIGN with a CM SERVICE ACCEPT, but will never send any CP-DATA. After some time, the BTS/BSC will close the channel again.
It is well within the authority of the network to decide which channel type to assign to the MS, and there is no technical reason why SMS delivery should not be done over the FACCH.