Call rings on bluetooth headphones, but when i pickup it defaults to the call being on the iPhone I have my iPhone connected to a pair of bluetooth headphones for listening to music. 

If someone calls while im listening to music the call will ring on the headphones, but as soon as i pickup the call, by using "slide to answer" on the iPhone; the call starts on my phone eg. through the phone's earpiece, i then have to manually select the call to be taken through the headphones. 

Is there anyway i can set the iPhone to continue the call through the headphones by default ? 

Im running ios 11.3 on an iPhone 7.

The bluetooth headphones im using are Bose quiet comfort 35.
The phone calls audio will be routed to the device you accept your phone call with. Use the play/pause button on your headphones to pick up, then the headset will be used for the phone call.