Yes, this can be done but there is a trick to it....
1. Edit your AD connector config and save/publish as follows:
{
"direction": "ex2in",
"internal": "contactInfo.phone_work",
"external": "User.TelephoneNumber"
},
{
"direction": "ex2in",
"internal": "contactInfo.phone_work.extension",
"external": "User.IpPhone"
},
2. Then go into the user's AD account and add the Telephone number in the designated field and then add the extension in the IP Phone field on the Telephones tab. Make sure to add both at the same time, do not save/apply until both fields are populated. After the connector syncs, you will see that phone and extension are both showing in PureCloud. The trick is they must be updated at the same time in AD otherwise one tends to overwrite the other.