There are a few settings to look at. On your S25, are you allowing it to automatically connect? In addition to selecting whether the connection should include calls and media there should also be a setting for whether the connection should happen automatically.
This goes along with B. Diddy's mention of other devices competing for the connection. Maybe remove auto connect from other paired devices, just to be sure they aren't interfering.
The next setting would be the head unit setting for priority devices. I don't know what type or brand of head unit the Volvo has but in my Toyota I have the option to select which device takes priority when connecting. I actually found this by accident when my son's device kept kicking me off whenever I would start the car. I'm sure the Volvo would have a similar feature to ensure the correct device takes the lead.
On that note have you removed everyone from the head unit and tried that way? With no other connection options will it connect then? If it doesn't connect with no other devices paired that could send us down another path of troubleshooting.
Have you noticed any pattern to when you're able to connect? I don't know, anything like, I can connect if the car is in the garage but not if I'm parked out front, or I can connect if it's me and my child but not if it's me and the wife. Since you say it connects sometimes looking for the commonality for when it does and doesn't could be key.
Have you done any testing? Maybe just sat in the car trying different ways to connect. It sounds like you've tried restarting the phone, but that was still hit and miss, it sounds like. One thing to try would be to close all background app before trying to connect. You can't try this in Safe Mode because SM disables connections so the only way to know if it's something like a bad app would be to close everything in the background, which this may have been what happened the times you say restarting the phone worked. The times when it didn't maybe that app reopened in the background when the phone rebooted. I've noticed with my S24U that even powering down the phone doesn't clear the background app list. I don't know if this means they are "still open" but the list is there.
The last thing to consider would be something is up with the phone. I know it's a new device but new devices sometimes have issues too. If you knew someone else with an S25U that you could get to connect a few times just to see if they experience issues the way you have. Which could point to some kind of compatibility issue between the head unit and the frequences or software used by the S25U or something. You've established it probably isn't the head unit since everyone else can still connect and it worked recently with another device of yours, pointing us back to the S25U device itself.
Let us know what you find and if you would answer the questions asked it would help us better help you troubleshoot this issue.
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