POTs line into Asterisk

I have a Grandstream HT-503 that is bringing my POTS line into my asterisk system. This works well. The one issue I am facing is now all calls are transferred to asterisk. I can easy solve this by remove the unconditional forwarding from within the Grandstream. Is there a way I can have the Grandstream alert a VOIP extension while still allowing the pots phones to ring?

you should check the configuration on the grandstream and your dialplan, you configure the grandstream as a sip trunk that goes into a context, then you have to check what are you doing in the dialplan, maybe you are answering the call before dialing the extension.