the calls don't come through your Voipo account. You can search for SIP scanners, or read a decent discussion here. The post deals with how to thwart them as well; however, unless you have unlocked (BYOD) device, you can't change SIP port or incoming dial plan (I don't know if such thing even exists on Grandstreams).

I would think that Voipo can set up the adapter to reject all calls that don't come from Voipo servers...