If they have travelled a long distance, maybe their phone simply needed charging, and maybe they are catching up on their sleep.
You seem to be panicing a little early, it’s only the first day, and in general we would only message an HO on the same day of arrival (or same day the HO left) if we were letting ourselves into a property, whereas you had a friend greet them, so they may have naturally presumed you would be in touch with your friend.
As for what to expect on the communication front, there are no rights nor wrongs if you haven’t already discussed it. Some HO’s don’t want any updates while they are on holiday, but a lot like an update every 2-3 days, and some daily, but if you haven’t already discussed how often you would like updates then you can’t expect updates from them.
Personally, if you want updates then I would pop them a message now saying you hope they have settled in nice, and ask if they would give you a daily update with photos, but if they don’t do that, then you can’t really hold it against them, because you should have let them know your requirements before the sit started.
But please relax because they’ve only just arrived today, their priority won’t be on messaging you with your friend greeting them, unless you had previously discussed that they would message you to let you know they had arrived. Their priority will be on your pet, where the pet food is stored, feeding times, where the poop bags or litter trays are etc, and where the kettle is if they’ve had a long journey, but also they will probably be sitting relaxing quietly with your pet so that your pet can relax in their company and get to know them.
They were delayed in getting to you, and perhaps you had no reason to think they could potentially be late, so I understand that has caused you to read more into other things, but it’s only the first day, so give them a chance.