I'm sure each of us have varying circumstances and issues we must address to mitigate risk and remain discreet.
There are a few reasons I use a physically separate phone.
1) I DO NOT want my civvie phone in a room with a provider.
This way, I can't accidently answer a call or ass dial someone from my civvie phone if it not on me during a session. Also, there's nothing for the provider to look at or snoop through while I'm in the shower.
2) As mentioned, the number is in no way linked to my civvie phone and can't be disclosed by service providers (in case of an LE warrant, etc.)
3) Less worry regarding civvie snoops (SO, girlfriend, etc.) My civvie phone is clean, search it all you want.
In my case, I'm able to keep my hobby phone safely "off-site". It is only turned on on hobbying days. I have a very specific ritual I use when "running" to a session. Civvie phone, ID, valuables stay in my car that is parked outside the potential range of LE surveillance. When arriving at an incall, I just walk up on foot from out of nowhere with a burner phone and some cash, that's it. No ID for her to look at, no impounding my ride if LE sting, no jewelry to steal or get lost in police evidence/check in, just the cloths on my back, a nearly empty burner phone and a couple hundred bucks in my pocket is all there is should something go sideways.