Telephone numbers

I've got to the stage with some of the structures on my layout where I want to add signboards to the commercial buildings. To add some life to these I'd like to add the firm's telephone numbers - but don't know in what format these would have been in small-town rural America in the 1940s. Is it likely they would just be a three or four digit number (or more), or should there be a two or three letter prefix for the town's exchange? 
 
Is anyone out there with long enough memories, or the specialised knowledge, able to help me with the answer, please?

Hi Giles
I use numbers for Colorado with the three letter prefix COL and then hyphen and then a 5 figure number (you could get away with a four figure one for a small exchange).  You could make your prefix up but by keeping the same format you would be in the correct era.

Hi Mike,
 
I thought it might be something like that - remembering the old tune PENnsylvania 6-5000. Just wasn't sure if all numbers were the same length or if small towns with smaller exchanges had shorter numbers.
 
I can remember growing up in the 1950s with a manual exchange where you asked the operator for the (four-fugure) number. Ten years or so later STD (subscriber trunk dialling) came in, and telephones suddenly got five figure numbers with another four digits for the town's exchange. With this in mind I wanted to get the size of the number right for the period I'm modelling.
 
Thanks for your input.