GSM CALL not getting through while ppp dial up is progress

I have facing this issue ,

-> I running gsmmuxd to create two virtual port
-> one Port (say HL0) for gsm -At command
-> Another Port (say HL1) for pppd

when i make gsm call on one part and other port call the pppd , it result gsm getting drop & received NO CARRIER.

And this is what unit…

This is HL8548 module