Germany to US should not be an issue. You may have issues with latency (ie: you say something and it will take the other side a few moment before responding... if you watch interviews with reporters in Ukraine on CNN for example, you will know what I mean). I would avoid VPNs for this type of stuff as it will increase the delay part and often will cause stutters and stuff like that.

Keep in mind, I'd probably try to avoid taking such calls as there may be security/privacy issues as stated by another member. There is legislation that is supposed to cover this (eg, HIPAA or PIPEDA or GDPR), This is why most physicians assign another physician to cover for them while away. There is encryption, but that often doesn't factor into these things.