In the telecoil (T/MT) programs, when should I adjust "Overall gain offset" or "Telegain offset"?
Use Overall gain offset if the overall loudness level of the program is too loud or too soft. Adjustments of this feature only affect the selected program. 
Use Telegain offset if the signal from the teleloop system (or phone) is too weak or too strong. An adjustment of this feature affects the T level in all teleprograms.