Yeah, I think she could have said "call me in a month" but I think she just thought Desmond was going nuts. It wasn't until she started to learn about the island and all it's oddities that she began to wait for his call. Although technically she saw him in California between giving the number and receiving the call.

I loved that line^^
One question though: Was Penny waiting for the phone call on christmas eve? Because if yes, it took her quite a long time to pick up that phone.

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- Does anyone know how to use a gun?
- I think you just pull the trigger.

- Hey kid, what's eating you?
- Nothing, he's at the top of the food chain!
(The Lion King^^)

I don't think she was waiting for the call in the sense that she remembered that conversation and was thinking, "I have to be available on 12/24/04." Because it didn't make sense to her. Why would Des call then and how did he know he was to call then. However, just in case she may have been around (didn't make vacation plans, etc.). Something obviously happened between that conversation (des asking for phone number) and them meeting again in L.A. to have Penny want des. back again. At that point she didn't really seem to care and mostly wanted him to leave so she could have her clean break. I think we're missing another piece of the puzzle here.

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...embrace the senile genius...
watch him reinvent the wheel...

requires Desmond's charm for fullest effect.

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"Bear...? Is that you?"-- Hurley S03E4