I've done my homework and searched on the website, and I could find a similar question: Should we cancel the Daily Stand-Up if we have another meeting during the day? but my case is different: my Project Manager is cancelling (or changing the time of the stand-up meeting the same day), not if we have another scrum related meeting, but rather if he (only him) has a call with a client or if he is busy with something else.
I know this is very wrong but I'm asking some help from the community to give him constructive feedback, ideally based on sources or articles that I can share with him.
I should also mention that I work in a global digital agency with clients across the world. In other words our PM cannot "afford" to ignore or move a client call as he likes.