Problems with external calls queue

Hello people,
I have a problem with call queues via a GSM module with a SIM card.

CentOS 6 + Asterisk + GSM Module Server ( DAHDI ) + 1 SIM

I 'm trying to set up a queue outside calls with agents , and this is my problem.

When I call the number of the SIM card in the GSM module I have :

If I call only with an outside number I have no problems , overseas phone rings hold music until the call is attended by all right agent, but when another outside number calls the internal SIM same in which the first call is on expected in the second phone a busy tone sounds and my call is cut ( does not enter the queue).

I’m looking for solution and can not find it anywhere.
Can anybody help me? (I’m a newbie).

Thank you.

1 SIM means one number, one number means one concurrent call. You will need more SIMS(numbers) in order to have more than one concurrent call in your system.

If you want an Unique DID but with many(+10) channels then you need to switch to another technology like E1 or SIP, with analog(in this case SIM) that will not work.

Thanks for your reply and sorry for reponder so late, was busy.
There is no way to put the calls are directed to the same number instead of waiting to put me busy line ?
Such as activating a mobile phone in settings or by pressing the corresponding key combination to activate call waiting telephone line .
I have activated call waiting on the SIM card in the phone if I call more than one person puts me on hold but not on the server , and not what the reason is , not if I have to change something or can not be done directly .
Thank you.