Cannot send SMS with Adafruit FONA

Hello,

I am using Adafruit FONA Adafruit FONA, for a long time. In the past I could send SMS just fine. I have connected it to the Arduino MEGA 2560 R3. I use Adafruit FONA library, configured to the right connections:

#define FONA_RX 10
#define FONA_TX 63 
#define FONA_RST 7

The problem now, is that when I try to send SMS to a cellphone, I get this error:

FONA> s
Send to #1234567890
Type out one-line message (140 char): hello world
	---> AT+CMGF=1
	<--- OK
	---> AT+CMGS="1234567890"
	<--- > 
> hello world
^Z
Failed
FONA> 
ERROR

I changed the SIM that I have inside with other 2 SIMs, in case there was a problem with the SIM card, nothing changed, the same problem…

Can anyone figure out, what is wrong? I also tried to read battery voltage—> everything OK, RSSI —> everything OK.

Thank you…

I also get this reply, when I try the network status:

FONA> n
	---> AT+CREG?
	<--- +CREG: 0,0
Network status 0: Not registered
FONA>