Between my wife and me, there are a few retired phones lying around! You won't get lab quality measurements, but it's great for before/after testing if you are making audio circuit or speaker changes. Of course, the Bluetooth connection and the phone's mic add their own curves, but it is still useful as a zero budget test setup! Good idea to use the newest Bluetooth versions that you have, though.
I have used this approach to generate some low frequency tones while testing changes to coupling caps and also to tweak a speaker enclosure.
It's interesting to see how much room acoustics mangle the frequency response of your console! This is a sweep from 20 to 20,000 hz of my SK111 in our living room. 'Course, I can't hear anything above about 9khz! The red trace is the max sound level after sweeping through the frequencies, yellow is background noise in the room.