Depending on the time of year, it may be hard to find a spot. You can't park anywhere, but there are designated spots to camp in. I went at the end of April and it was packed. 

You might be able to park overnight and leave early in the morning in some areas, not sure how much they patrol the area. 

No cell coverage for T-Mobile, but I used my WeeBoost and got two bars, enough to make calls and texts.  

Use caution when traversing though the area to find a site if you have a normal sedan or normal vehicle height. The road is bumpy and there can areas that are steep. 

My Van got stuck through a slot canyon as I was trying to find a site. I didn't have my recovery tracks, since it was a last minute stop on the way back from Mammoth. 

With the help of a nice nearby camper who had a wench, we were able to free my van in three hours.  It was around midnight, so I ended up just driving out and parking in Lone Pine to sleep for a few hours.