Arduino + Sim800L (AT-Command,Receiving SMS) Help

Its my first post here ...

Finally I'm able to run arduino mega and gsm 800L...On 115200 serial , 9600 gsm bauds..
Now few problems..
Im able to

-Send SMS
-retrieve received SMS to serial window by from one index location at a time

-Only some AT commands responds on serial
Like AT ,ATE ,ATA,ATH,ATE.. While if a call is incoming , serial window Shows "RING"

ANY other commands like AT+xxxx or ATDxxxxx don't work and give me error

For e.g :
ATD12345678; i press enter and get the following reply
"AT+
ERROR"

All i wanna to do is few simple tasks using Arduino mega+sim800L :

1-Received command("ON/OFF") through SMS and Reply confirmation SMS
OR
2-Make a call when some pin goes high or low

Also how to extract info/command from sms , because ON SERIAL MONITOR it show sender number , at command and time then it shows actual SMS..

Ive used most of the libraries available on internet but none works , and the only working one is cristianteib(serial,readsms,sendsms) ..
Also in his library functions there is one "void callNumber(char* number)"
How can i use that?

These were working codes for me :

https://cristiansteib.github.io/Sim800l/

Share how all is wired, powered, how you get info to and from serial etc

Arduino pin 10--TXD of GSM , 11--RXD of GSM..
Gsm is powered with 3.7v 1.2A battery (mobile phone battery) GND is common..
The link i posted have the codes from which im able to get and send sms ..