Hi ms hd,
I agree with you. Giving priority to the phone is essentially allowing another customer to 'cut in line'.
In your case, i would use the voice mail system to explain that you are 'unable to help because you're helping another customer right now, but please leave a message and we we'll ge back to you as soon as possible.'
It would be most preferable to have someone who is not working the cashier as the designated phone answerer, but in shops this isn't always possible because of few resources.
Just my opinion.
Kind regards,
Erwin Steneker
http://www.customerservicepoint.com