hi,
technically both wired and wired are feasible. you may want to consider to go with a majority of the users on wired phones, and wireless phones for users with special needs.
for me, a wired desk phone would seem more practical to a normal office worker.
either way, there is a need to deploy the infrastructure for the voip devices to work on, if it is not present;
- wireless equipment, APs, controller, authentication server (for wireless clients), or
- PoE switches and cabling for wired phones
regards
you may wish to repost under "ip telephony" for better response.