The Canadian prime Minister went and spoke directly to Trump.
And Trump laughed him off.
And how do you know the ambassador hasn't already had those meetings?
Canadian officials have been in communication with American governers, senators, congressmen and even mayors to discuss the situation and what sort of response they might expect.
A big part of the problem is that Trump doesn't listen to the advice of anyone once he gets some stupid idea stuck in his head.
But it's all shit talking and "jokes". Summoning the ambassador, the Canadian government as an institution can officially demand an official response from the US government as an institution. Sovereign state to sovereign state. This is serious business not a social media exchange.
It would basically demand the US government to produce its formal demands of Canada. Anything else should be ignored as farts and white noise.