Bessel filters: linear passband phase characteristic only for low pass filters?
Filter types other than low pass (high pass, band pass, band stop) are obtained from a low pass prototype filter by frequency transformations (such as $s\rightarrow 1/s$ for a low pass / high pass transformation). These transformation destroy the optimality of the phase response. Nevertheless, filters obtained from a Bessel low pass prototype filter usually exhibit a passband phase response that is closer to a linear response than the phase response of the other standard filter types (Butterworth, Chebyshev, Cauer).