I am so sorry to hear about your mother-in-law. May God console your husband and the entire family. May they be comforted and know no more sorrow.
Judaism teaches that that in general we should try and “judge others favorably”. This means that although we see someone do something that looks wrong, we should not be quick to judge the person as errant, but should try to imagine what justifiable reason the person may have had for the action.
Having said that, from you write, the case you describe seems like one of clearly inexcusable behavior. What you describe at the service sounds like the insidious encroachment of modern day technology into our lives. I cannot imagine why he felt it okay and necessary to speak on the phone during such a ceremony. It was obviously disruptive, annoying and disrespectful.
The only thing I can think of is that he didn't realize what he was doing, but I find that too far-fetched to consider. Perhaps you or someone else who was there should speak with him in a tactful manner and make him aware of the problem. Perhaps he will not repeat it.
I personally would suggest that cell phone use be banned at such ceremonies, as they are at many less important and holy events due to the disturbance they present. May we all share only happy occasions in the future.