I am sorry to hear your Buds Pro are not working for calls on the S21 Ultra. If your Galaxy Buds, Galaxy Buds+, Galaxy Buds Live, or Galaxy Buds Pro disconnect from your phone often, it may be due to a poor Bluetooth connection or because you are wearing the earbuds incorrectly. You should check that the earbuds are properly inserted into your ears, that your Bluetooth connection is secure, and that your earbuds' software is up-to-date. Try these steps here: https://www.samsung.com/us/support/troubleshooting/TSG01208805/
Please let me know if this helps.