Insert line using UpdateConfig

I am using Asterisk 13. I am trying to add a new line in extensions.conf using AMI. For this I am using the “action” “updateconfig” explained in the Asterisk wiki:

Wiki Asterisk UpdateConfig

The line in extensions.conf that I’m trying to add looks like:

exten => _X.,50005,Dial(${dev},20)

Using telnet to port 5038 I enter the following:

srcfilename: extensions.conf
dstfilename: extensions.conf
Cat-000000: cat_prueba
Var-000000: exten
Value-000000: _X.,50005,Dial(${dev},20)
Line-000000: exten=>_X.,50005,Dial(${dev},20)

This results in:
Response: Success

Even if the answer is “sucess” the line is not added. How can I do it? Any suggestions?