I'm sure there was a time when it was considered rude to use the telephone for a message like that, and people would be expected to deliver it in person.
Times change, and these times in particular are difficult. My niece recently had a baby and her husband was told that if he left the hospital he wouldn't be allowed back in, because of the virus. Maybe there was something like that going on? If your son's battery was flat he wouldn't be able to get away to charge it.
In any case, my advice, for what it's worth, is not to say anything about this offline. You are right at the start of a new stage in your relationship with your son, his partner and your lovely new grandchild, and the very last thing you need is to be accused of making it about you. I don't mean that unkindly at all, but I do think it's a case of least said, soonest mended.
Enjoy being a granny - I'm very envious of you having a new baby to coo over when you can get to see the new family in person.