You can transfer a call internally (to another person in your company) or externally. An attended transfer, also referred to as a warm or consultative transfer, is a method of transferring a phone call where you speak to the person you are transferring to before making the transfer.
The caller is placed on hold while the call rings the other person's phone number. When the other person answers, you can explain who is on the line and why you're transferring them. This gives the other person time to prepare for the call before receiving it.