High Latency Communication with Potential Thesis Supervisor I'm currently pursuing my Masters in Switzerland and got in touch with a Professor at a major UK university a month ago via E-Mail for a potential thesis project. I immediately got a positive reply mentioning my interesting background and the good fit with his current research.

Two weeks ago we talked on the phone for a little more than an hour. It was almost exclusively him explaining three research projects that would fit my background and interest. It all sounded very positive and ended with his request for me to think it over and tell him as soon as possible if I was interested in any of the projects, as he was "super busy".

I decided on one of the projects that I really liked and wrote a nice E-Mail the same day.

I have not heard back from my potential supervisor yet (2 weeks in now). I wrote a brief follow-up E-Mail one week ago, asking if he knew whether he wanted to go ahead with our collaboration. This week he will be attending a conference abroad (according to search results on Google), so surely he is very busy.

While I have had Professors not reply to E-Mails about thesis opportunities in the past, I am confused both because of the good conversation we had on the phone and what we had agreed on in that conversation. 
After all, if he really has lost interest in a collaboration, he could easily drop me a line via E-Mail.

Right now I think another E-Mail by the end of the week would be the only option - reaching out by phone is difficult, because he seems to rarely be in his office. How do I come across without making the impression of being pushy or annoying?
They sound sincerely interested in working with you. There's no right answer to this; you're right that there's a balance between staying engaged and giving them polite space to reply. It's likely they will be coping with various life events, not just work; who knows what may be filling their time and attention. It's normal that people may be responsive in one period and then less so.

I recommend perhaps lengthening the time between check-ins, so 1 week for the first follow-up, and then 3 weeks, and then maybe 6; and then stop. Alternately, you could call as much as once a week, if you don't leave a message. Most of the risk of too much contact is because of making them read or respond to a message, so if there isn't one, that's less of an etiquette problem. Ultimately, there's no way to control whether they experience this input as pushy or annoying. I would also continue exploring other potential advisors.