SIP phones, either physical or software based, offer distinct advantages in a modern communication setup. Not surprisingly they are favored by most users
Traditional analog phones can't be plugged directly to your IP network since they have analog RJ11 connectors instead of IP ports.
To address this you will need to purchase an ATA - Analog Telephony Adaptor. The ATA has both an IP port and RJ11 (or other analog) connections.
It acts as a bridge between the IP network and the analog phone, converting digital data to analog signal and vice-versa.
The VoIP features that the analog phone will be able to perform depends on the ATA itself. Most basic functions will be covered. Advanced users or those dealing with large call volume may require an actual SIP phone.
An ATA can also be used for connecting other analog devices like a fax machine or a Door Phone. These may have an RJ45 connection used for analog data transmission.
Not only a good ATA is cheaper than a basic SIP phone, but business grade models usually comes with at least two analog ports for connecting two or more analog devices, making it an even more economical solution.
If the office location of your analog phones is not near an IP port, an ATA that can connect over Wi-Fi will easily solve that. However, a physical IP connection should provide a better call quality and is inherently more secure.
Once you connect the analog phones to the ATA they still need to be configured to recognize the relevant server and user settings. With some ATAs this can be done by the process of auto-provisioning. With others you would have to login to the ATA's IP interface and enter those values manually.