To add to Steffan's excellent answer, simply connecting the phone to the WiFi cannot, by itself, add a certificate (CA or otherwise). However, if you enable your employer's mobile device management (MDM) on your phone - which may be required in order to do things like access your company email account or connect to internal (intranet) web pages - then the MDM software may be able to install a root CA certificate.
The details of what MDM makes possible depend on the phone operating system and the MDM software used, but all major mobile operating systems support at least some MDM. On the other hand, just because MDM is in use doesn't mean that a CA certificate has been installed. MDM gets used for many other things as well. Among the most common are enforcing the use of a PIN on the phone, requiring that the phone's data be encrypted, and giving the company the ability to remotely wipe the phone if it gets stolen (or if you leave the company while there are sensitive documents on the phone, or similar).
To know what MDM your employer is using (if any), I suggest you take a look at the IT / helpdesk web site and see if you can find an answer. If not, try asking them about it. Alternatively, simply don't add anything from your employer to the phone - no certificates, no company email accounts, no company apps, etc. - and keep it as your personal device. You still might not be able to use the internal WiFi for TLS (HTTPS, etc.) if it requires a CA certificate, though...