Digital voice has an inherent delay. The more conversions from analog to digital to analog to ..., the more the delay. Listen very carefully without Bluetooth and see if there's just a little delay. Then use 2G calling with Bluetooth and see if there just a little delay. Then add them for Bluetooth using wifi. (My VoIP system, when the company was starting up, had over half a second delay to my office - their a-d conversion plus my office's d-a conversion. Both eventually got "modernized" and the delay is barely detectable.)
Not much you can do about it until we have much faster converters and less latency on wifi.