It would be recommended to have the earbuds evaluated by a certified technician. Please send a private message with the full model code and the serial number from the bottom of the charging cradle. http://bit.ly/2XERsky
This also just started for me this week. I saw another post (yesterday) with the same issue. This does not seem like a hardware issue, but perhaps a software bug that has started for multiple people in the last couple of days.
Buds clean, unpaired and repaired, no physical trama, and all other sound works perfectly fine except for phone calls.
If all troubleshooting fails, the next steps are repairs.