Using invfreqz() without knowing the phase response?

[OCuanachain, Oisin (Oisin)]

Hi, 

I would like to use invfreqz() to create a digital filter [b(z),a(z)] as a 
model approximating some empirical data. The problem is that I only have the 
amplitude response of the system not the phase response. This shouldn't really 
be a problem as I am only interested in the amplitude response however to apply 
invfreqz() I need a phase response. Eg
Amplitude response = A(w)
Phase response     = phi(w)
Then 
H                  = A(w).*exp(-1*j*phi(w))
Then [b(Z), a(z)] = invfreqz(H,w);

So what I really want to know is, what is an 'easy' dummy phase response I can 
use for phi(w) which will maximize the probability of invfreqz generating 
digital coefficients which will match my desired amplitude response well ?  Or 
to put it another way what phi(w) can I use to ensure that invfreqz() doesn't 
run into problems. 

I realise I could probably work this out myself by reading the source code for 
invfreqz() to see exactly what algorithm it uses, but if someone else already 
knows the answer it would save me the trouble :)

Thanks, 

Oisín.