Lee,
I too used to give quotes out over the phone, when I first started, not too long ago. I made some bad mistakes.
Now I've learnt from them, it's rare I give quotes out without viewing the property. In fact I'll only do it for an Army married quarter estate where all the houses are exactly the same.
One of my most favourate telephone quotes was when a woman described a normal three bed semi. I gave her an average quote. Then when I turned up it was a mini mansion, leaded front and back.
I might tell a potential customer who presses me for a price that our average for a three-bed semi is £7.50 - but if yours is harder or takes longer than our average, then we'll charge more.
What you should do is, over the course of your working week, note down all the addresses that require quotes and then go out on a Saturday morning and price them up.
Then, phone the potential customers that weren't in and tell them your quote. On another thread, you said it takes too much time to view and then wait for an answer; but remember, it might take say, 20 minutes to travel, price the house and speak with the customer; but you may have that customer for the next 20 years and get thousands of pounds out of that job (over the years, obviously).
Don't price blindly is my advice too.