First of all, thanks for asking! I’m always happy to answer questions, so don’t worry about annoying me— I love getting messages:D
Now on to the question!
I don’t think the voicemail is ever coming back. It’s one of many loose threads that could plausibly return, but I’m not sure what the show would gain from bringing it back. Alicia already knows it exists (“On Tap”) and she seemed to accept Will’s explanation of its contents (“Net Worth”), so there’s already been some resolution. I can’t think of a way the voicemail could return that doesn’t involve Eli telling Alicia he deleted it, and I can’t think of a reason Eli would tell Alicia.
Further, I think the show implicitly addressed the first missing voicemail by structuring The Last Call around a second missing voicemail. It’s true of both voicemails that Alicia becomes fixated on the content of the missed message— in 2x14, she tells Owen about the first message, discovered several episodes earlier; in 5x16, she goes on a quest to find out what Will wanted to say. The circumstances are entirely different, but the guiding principle is the same: Alicia “hates missed connections and that stuff” (2x14). Both situations, too, involve the idea of something being “romantic because it didn’t happen” (1x18, Alicia reflecting on her time with Will at Georgetown). The first voicemail creates a thread that hovers over Alicia’s (unhappy) marriage to Peter for much of season 2 and pushes Alicia to come as close as she ever does to verbalizing her love for Will (“You’re in love.” “I can’t be.”). The second voicemail drives Alicia to fantasize that Will was calling to confess his love, apologize, and pledge to be with her forever. As the writers tweeted, “In the absence of facts, you have to create your conclusion.” Alicia needed some sort of closure, so she got as much information as she needed (hearing that Will was angry at someone else and possibly hearing that Will was interested in seeing pictures of Finn’s son, making him more of a family man than she thought) to allow herself to believe in the fantasy that would give her that closure.
The message Alicia wants to hear in 5x16 is quite similar to the message Alicia never hears from 2x01— what’s different is the context. In 2x01, if Will had been speaking to Alicia when he said, “my plan is I love you,” Alicia probably would have continued to insist that she needed an actual plan, or at the very least, that she needed Will to understand why a plan was important to her (his offer to meet to make a plan conveys that he’s understood that she needs a plan). But in 5x16, Will is dead. Alicia doesn’t need a plan then. “Forever” is enough to satisfy her. In 2x01, she knew the romance was there and the next step was the plan. In 5x16, she’s wondering if the romance is still there, but the plan is irrelevant— it’s illogical to plan to enter a relationship with a dead man. This is why I say the 5x16 voicemail stands in for the 2x01 voicemail: the 2x01 voicemail is outdated. Will’s declaration of love from then, if Alicia heard it now, would raise more questions than it would answer. The 5x16 voicemail is an update of the 2x01 voicemail in that Alicia needs to believe that in this context, Will would profess his love for her. She needs to believe that, after all the fighting and anger, he still loved her. She needs to believe that he didn’t die angry at her, and she needs to convince herself that it’s legitimate to grieve Will as intensely as she is. Hearing the 2x01 voicemail wouldn’t accomplish any of that, but creating a new, similar voicemail does (and gives the show a nice thematic link, since missed voicemails fall under the umbrella of the “bad timing” which has characterized Will and Alicia’s relationship from the start).
This is a long (hopefully not too confusing) way of saying that I don’t see a narrative purpose for bringing back the old voicemail, especially since we’ve already seen Alicia deal with a new voicemail. Alicia’s come to terms with it; Alicia’s allowed herself to believe that Will was still in love with her (which, as I said above, hearing a voicemail from years and years ago, a voicemail that predates the A/W affair during which Alicia clearly got the sense that Will loved her even if he never declared his love quite so passionately, would not help much with). The writers seem to want to shift away from the idea of Will’s death strictly affecting Alicia’s love life, and bringing back the voicemail would work counter to that strategy, assuming the writers continue to go in this direction. The only level I can see the voicemail really working on is within the Eli/Alicia relationship— the Kings have mentioned before that “The only thing that hasn’t been addressed, to our minds, is Eli’s duplicity in erasing the voice mail”— and I’m not sure why they’d choose now to upset that relationship. The dynamic between Eli and Alicia has evolved so much since early season 2, and I think there’s more to be gained from showing how they relate to each other now than there is to be gained from upsetting the relationship— one of the few left in tact after season 5— to go over something old. But the Kings tend to operate under the principle never say never with these things, so I’ll go with that as my final answer, too.