Using more than 2 "line" keys on Mitel 6869i

I want to use 4 “line” keys instead of 2 on a Mitel 6869i
The phone comes with plastic keys name L3 and L4 and one of our customers wants to be able to manage 4 calls at once.

I did change the key configuration in the template by using the following settings:
hardkey3 type: line
hardkey4 type: line
hardkey3 line: 3
hardkey4 line: 4

I can use the keys to pickup a line (it shows L3 and L4 on the top of the screen) but as soon as I try to dial something, the phone just hangs up.

For incomming calls, if I have 2 active calls and a 3rd one comes in, it goes to the mailbox, playing the “on the phone” message (like the line keys 3 and 4 cannot receive calls).

I’ve been searching all morning but I cant find a way to make it work. Is there a hidden setting somewhere that limits the extensions to 2 simultaneous channels ? I did change the “Outbound Concurrency Limit” to more than 3, both in the extension’s settings and FreePBX advanced settings.

Ok I have my answer.
I have to manually configure accounts 3 and 4 on the phone and it works. For a reason I ignore, the account 2 doesn’t need to be configured in order for the second line to work.