Everyone is put on the spot at some point or another. Here is what Ms. Emily Post herself says about invitations over the phone:
If you are extending an invitation by phone be direct. Something like "Hi Suzie, we are having a few people over Friday night for a game night, can you and Steve make it?"
Suzie can either accept or say "I'm sorry, sounds fun, but we already have plans Friday night."
"I'll let you know," by itself is not an appropriate response. This makes it sound like you are waiting for a better invitation to come along. The only time it's acceptable is if it is followed up with an explanation like, "I'll let you know after I check with Steve to be sure he is free Friday night," or "We have tickets for a play Friday night, I'll let you know if I can exchange them for another night."
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