Actually, they did have a retail value. Right at the time of the breakup, when you were allowed to purchase your own phone, you could purchase the classic bell phones.
Trimlines were $85.00
Princess phones $65.00
Regular wall/desk phones (500, 554 series) were $55.00.
A year later, the price went up but I can't remember what they cost.
They still have a lot of value. If it's in good shape you can easily get $30 or more. They were built so solid you can't destroy them!
Remember that when they made those phones, they had a lifetime guarantee. The phone company would simply replace a phone if it became defective, and refurbish the "guts" into another case. They had a plant at Western Electrict that did nothing but recondition them.
Serial number won't help me to identify it. What't it look like?