Recently I got a chance to attain FB telephonic round, below are the two questions I had been asked. Interviewer was very good and I must say it was one of the best interview experience I have had so far.
I could code both of them on time and I was able to explain the flow with cornner cases. I had missed few cases which I fixed on the go and he was fine with that.

1. https://leetcode.com/problems/add-binary/
2. https://leetcode.com/problems/valid-number/ (not exact one but some variant of it)

Analysis :

1. I felt I was lucky as interview did not spend time on intro but he directly jumped into questions which provided me more time other wise in general 5-7 minutes goes into exchanging intro.
2. I intentionally proposed different possible cases which idicates that I have wider vision to the problem. In some cases he mentioned due to lack of time we can skip it or suggested I can assume in my code. But I clearly see he is noting down all those conversations, that went in my favour.
3. If any cornner cases are missed don't get worry but try to fix it on the go. That give a indication that you are good in catching bug in your code. Which is again a possitive trait.

I finished it almost on time (40 min - 5 min normal chat ) got mail yesterday (after 6 days anxious wait) that they are considring for next on-site rounds.