Dinstar FreePBX Call Waiting problem

Hello, we have a problem with our phone system setup.
Setup consists of GSM Gateway Dinstar UC2000 with SIP cards in it. Call is routed from Dinstar through SIP trunk to FreePBX and then to Zoiper Softphone.
Problem is that Call Waiting is not working as intended.
Say first call is routed from Dinstar to Zoiper. If the second call is coming on the same number, then the first call is terminated and put on hold while second call is ringing, which is not acceptable to terminate first call.
Our goal is that second call should not terminate first call but appear in zoiper as missed call.
I should mention that Call Waiting works in case Zoiper is connected directly to the Dinstar.
Also it is working in extensions inside FreePBX.
It works even in a case when first call coming from Dinstar and second from other extension.
But in case when both calls are coming from same SIM number from Dinstar, it does not.
Please help to resolve the problem.

For reference this has also been brought up at Call Waiting Problem - Endpoints - FreePBX Community Forums

Yes, thanks. Any ideas?

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