Start with software, add hardware later
A softphone on the computer people already use costs nothing and works in minutes. Most businesses find that is enough, and buy desk phones only for reception or a shared area. Starting with software also means you find out what you actually need before spending on handsets.
One number, several devices
An extension supports three simultaneous registrations, so the same line can be live on a desk phone, a laptop and a mobile at once. All three ring; whoever answers first takes the call. There is nothing to forward and nothing to remember to switch over at the end of the day.
What to buy if you are buying
For most desks, an entry-level Yealink or Fanvil handset with two SIP accounts and a colour screen is enough, and costs less than a month of a typical hosted-seat licence. Spend the money on reception instead: a phone with a proper expansion module makes transferring calls far quicker than any software equivalent.