Agents Login Extension...(!?)

Hello to the community! I have a problem (or something misunderstanding) about queues. I declare 2 of my extensions (01 and 03) as agents.
In agents.conf file i wrote…

agent => 9991,1234,agent one agent => 9993,1234,agent two
In extensions.conf file…

exten => 88,1,AgentCallbackLogin(9993,,${CALLERID(num)}@numberplan-custom-1) When i call from extension 03, so the third parameter should be 03@numberplan-custom-1. It says that it is not a valid extension. But i use the 03 extension, as SIP phone to make and receive calls! Why is this invalid?

I tested having a new extension in extensions.conf for example

exten => 66,1,Dial(SIP/03) exten => 66,2,Hangup()

and it works! But it isnot a solution but another problem. I want to use my existing extensions as agents’ extensions… What i am doing wrong?

Thanks for your time,