Did you mean that you tested part of the overall call path by calling the European apartment's telephone number assigned to the OBi 110 from some unrelated telephone number, not his nursing home cell phone, and it successfully called the pre-defined forwarding number (your US cell phone number), and that part of the call path works reliably with two-way audio? If that's what you meant, then the problem is somewhere between his Panasonic phone and the smartphone in the nursing home.
IF that was what you meant, you could make things simpler, more reliable and more direct by replacing the Panasonic phone and smart phone in the nursing home with a simple, big-button cell phone. It's best to keep things as simple and user-friendly as possible for seniors, to reduce stress and anxiety and frustration.
For example:
https://www.snapfon.com/Note: that website describes a bundled telephone plus a mobile phone plan, and an optional emergency response service, but you can also just buy the phone by itself with no service. It's a quad-band world phone, so all your father would need to do is swap out the SIM in your smart phone, assuming they are both standard-sized SIMs. The other advantage of this would be to eliminate the audio degradation added by the Panasonic cordless-->Bluetooth-->smartphone kludge, and replace it with a phone designed for loud and clear, senior-friendly audio.
If that is not what you meant, please try to explain further.