I have 2 suggestions for this.
1. P.O box - that way you use it only when needed and at the post office most convenient to you.
2.Here`s a strategy that works well- Show your interest in cars and take your SO to a car dealer on 1 weekend on the pretext that you like checking out new cars and test driving them. Take her a 2nd time and then a 3rd time .
And everyone knows the dealer salespeople are very pesky. They will call you,hound you,pound you incessantly inquiring whether you are still interested,if they can make a better deal or what they need to do to earn your business..
And so in order to avoid these pesky calls from these salespeople,you do not want to give your real cell number to them as they would call you repeatedly and irritate you- similar to telemarketers.To avoid these irritating salesguys calling you all the time, you prefer a prepaid cheapo phone whose number can be given to every salesperson. That way you can talk to them whenever you want to--about the new cars,new deals,new leases etc etc--this should be the explanation given to your SO!! And she would never doubt it.
Plus an added benefit--by the 3rd time she is bored and wouldn`t want to accompany you again.Make sure these 3 times you are at the dealer for atleast 1-2 hrs. So the 4th time you go-she is not interested,she asks you to go by yourself...Now you have 2 hrs+hobby phone==time you can spend with a provider without having to explain the time period.
Happy car shopping !!