Ding ding ding
Control channels pass info to/from the phone, handle handoffs between towers, etc. they don't carry voice traffic. SMS is sent over this control channel when it is available.
All the frequencies for voice can be jammed up solid and sometimes you can still get an SMs through
Correction, there are frequencies, and there are codes/time chips (depending if its CDMA/TDMA)...so all the frequencies could be used, but the cell could still be at low usage.
Frequencies are typically allocated (in groups) to ensure minimal cross/talk co-utilization tower to tower, time/code division within a cell for all the packets.
Otherwise, you are exactly right. That is also the reason why SMS is character limited as its limited to the size of the corresponding control packet length.
So what you meant was that "all the bandwidth for multiple calls/data transmissions can be in use, and you can still get a SMS through"
The reason why is those channels are designed (and technically always are) serving the maximum number of devices in that area.