Personally, I don't quote over the phone.
I normally charge more for a first clean and it depends how filthy the windows are, as to how much extra I charge.
It goes like this:
"OK Mrs Smith my regular price, every 8 weeks is £22. And remember that's every 8 weeks, not every month.
However, your windows are particularly dirty and the first time I clean them, it's going to take me longer to really get them looking good. Would you mind if I charge an extra £8 to cover the extra time involved?"
I've never had a custy say no. If they did then I would politely walk away.
As for messers on the phone, you need to pre-qualify them a bit.
1. Ask where the house is (is it a good area or not)
2. I always ask 'Do you want a one off clean?' and then I don't say anything. If they say yes, then at least they are being honest and you can decide if you want to do it. If they say they want it regular, (which is what you really wanted to know), then you can normally tell by the way they say it, if they're genuine or not.
3. I always throw into the conversation 'so have you just moved in or...'
Some will say they have been in a few months, some will say 'no my last window cleaner just stopped coming'! I am wary of these. Some will finally admit they are students and are moving out soon.
4. If they push you for a quote over the phone I usually say "I'm sorry I can't quote you over the phone but I do need to see your property. (and then I bullpoop a bit) It's not just about your windows, I also need to check that I can park my van safely and make sure I have good side access to the rear of your property and it's safe to walk around and also make sure there are no health and safety issues. This usually does the trick.
5. When I go and see the customer I usually spend a couple of minutes having a friendly chat (all smiley etc), I show them my nice leaflet, from my nice van, wearing my nice logo'd shirt, I tell them about the pole system (in brief), tell them I clean all the glass, frames, sills, front door and back door and door surrounds. I text them the night before, so they know i'm coming and they have a range of options to pay. Also, if they have an awkward window, or the ones above the connie, no problem at all!
And then I add... 'I do provide a quality service and a quality clean but I do charge a small premium for it!
So now the customer is expecting a high price to cover all this extra waffle i've just mentioned. So when I give them my price (which is probably average for the area) they now think i'm good value. Happy days
None of it is an exact science, just do what works for you.