You can ask the FONA for the signal strength with the command i. The reply is a number, but you can convert it to dBm. Try to have the signal strength higher than 5 in order to make calls, SMSs, etc. In this case, I've got a 10
If the FONA has good signal it will immediately try to locate a cell tower and register to it.
You can check the status of the network with n
Once it's Home Registered, give it like 5-10 more seconds before trying to access/send SMS's or phone calls.