You are probably referring to the meaning of _dürfen_ that is listed in the [Duden][1] under 1.e.

> es ist wahrscheinlich, dass …

Your example sentence would be written as:

> Du dürftest mich am Morgen angerufen haben.

To construct sentences that way, you use the _Konjuktiv II_ of dürfen (see the [canoo table][2])


  [1]: https://www.duden.de/rechtschreibung/duerfen
  [2]: http://canoo.net/inflection/d%C3%BCrfen:V:haben