BLF for GSM gateway

Hello,

we have Asterisk with 8-port GSM GW.

In this example mobile numbers will be: 111, 222, 333, 444... (company employees, 30 mobile numbers in total)

When the call operater answer on incoming call, she transfer to mobile 111 trought GSM GW (and now one channel is busy).

We want to see on our phone (BLF) that one GSM GW channel is busy. For example 111 is speaking.
For next call operater transfers to mobile 333... and so on

Now we need information on phone (red BLF) for number 111 and 333. When 111 or 333 hangs up, BLF will be green.

So far we have information on phone that we have channel on GSM GW busy but we don't know which user is speaking.

Any idea?

Note: it doesn't matter which SIM card GSM GW is using