You can't plug IP phone it PC, you may connect PC to selected IP phones models with integrated 2-port switch. The main LAN switch may be manageable and may support VLANs, it is recommended if you want to split the data and voice LAN for security and/or quality reasons.
It is equivalent to have separate LAN switch ports and cables for PCs and IP phones.
Otherwise PCs and IP phones will not use VLANs and main switch could be a non-manageable one.
PoE is more expensive, but will eliminate the need of power adapter for each IP phone, plus you connect PoE switch to an UPS easier, than each phone separately. Using PoE switches is independent solution.
Using IP phones or IP (or analog) DECTs is third independent solution, depends whether you want corded or cordless phone on your desk. DECT will give you more mobility, depending on model.
Also you may consider using physical (hardware) phones, i.e. desk IP phones, DECTs, against using softphones on your PCs / notebooks or on your mobile phones.
So, there is no single answer to your question, depends what you prefer to have. All solutions will work and can be combined also.
