The use of the 2nd (cloned) line

I switched over to Voipo not too long ago. I have the Grandstream adapter with two ports. It's my understanding that it's the same line, but they operate independently. I have two phones plugged in. One of the phone has caller ID. I don't use that as the only phone because the phone volume stinks and I can barely hear anyone. The phone I use to make calls and answer calls is an old Conair phone which works perfectly--but no caller ID, which is why I have the second phone plugged in.

Now, every once in a while, I will get a phone call, and I will answer with the Conair phone, but I don't get connected. Instead, the other phone keeps on ringing. So I hang up the Conair phone, trying answering with the other phone that has caller ID, and I still don't get connected. When I hang that phone up, it starts a series of really fast rings.

I can't use any phone until I reboot the Grandstream adapter when that happens. It seems to happen (I think) when I get a call from certain mobile phone numbers. Do you think it's having two phones plugged in? Or is it one of the phones itself? Or the Grandstream adapter?

In the meantime I've unplugged the phone with the caller ID so I have only the Conair phone plugged in to see if I will have trouble answering a phone call with only one phone plugged in.

Just an UPDATE: Ever since I have removed the second phone on the other port, I have had no problems. I guess for some reasons it just didn't like it. Oh, I now have my phone with the caller ID plugged in as the solo phone. After something like 15 years, I just noticed a switch on the side of the phone that says Normal:Hi. For some reason I thought it was referring the the LED caller ID readout, but of course, it was referring to the volume! So now I can hear perfectly when I'm on that phone.