D-link forum

Hello,
I'm trying to use my D-Link DSL-2680 router with my VoIP phone system. Phone get registered fine and I can make outgoing calls as well. Incoming calls also works if I receive a call soon after making an outgoing call.
But if the phone is left idle, without any incoming or outgoing calls, I cannot receive incoming calls.
When I contacted my VoIP provider, they say it may be how NAT and SIP ALG setup on my router. I'm not very good at router technologies. But it sound like when the phone register with VoIP provider, router supply the initial port numbers. But then router changes the port number later on, that's the reason why my my phone cannot receive incoming calls.
Not sure whether this is the reason or how to fix it. Any help or advice on this is greatly appreciated.