Number of phones?
I've tried it both ways, and I prefer using just one phone too! I have a very loud, specific ring tone just for customers so I can tell when a customer is calling and it's loud enough to hear when I'm on a mower.
2009 Nissan Titan pullling a 18 ft PJ Trailer
2011 Kubota ZD326S (60") - 26HP Kubota Diesel
2010 Bob-Cat FastCat Pro 48" w/Accelerator (20HP Kawa)
Non-commercial 21" walk-behind
Blowers (Stihl BG85/Husqvarna 170BT)
Trimmers (Stihl KM110 & FS90, Kawa Multi-Tool)
Chainsaws (Stihl MS180 & MS362 w/20 & 24" bars)
& misc hand tools & equipment