If I have to use speakerphone my Polycom is a world better than any cell phone. Headset options are infinitely better than Bluetooth options. Back in the day I preferred a wired headset if I was going to be on the phone all day; now I have a wireless Plantronics since I'm not on the phone as much. The desk phones always work and work well. I notice whenever I'm talking to someone that has VOIP or is on their cell: the worst is when there's a one second lag on the other side's VOIP handling, it really screws up the flow of conversation when both ends end up talking over each other. I guess if you're both on VOIP and have the same amount of voice lag it probably balances out, but it's really pretty much crap when one side is trying to do something "modern" that sucks.
Yes, there's ways to set things up properly, but by and large all these alternate methods end up falling short more often than not. My office phones are VOIP Poloycoms and they work great, but they're on the LAN where every detail can be configured properly.
Plus my work phone is for when I'm working. Call it for work. If I really want to or need to be reached for work things on my cell the desk phone can be set to forward calls.