I don't think Mike is Spamton's mysterious phone benefactor, I believe that the garbage noise comment from the Addisons gives us every reason to believe Spamton was speaking directly to Gaster or whoevers making people schizos. Mike is referred to by Spamton with language that suggests he was once a friend and perhaps even a rival that left him behind once he fell from grace. He's probably just referring to another character outside of his Dark World's context like how he also mentions Jevil and how Jevil foreshadows Queen
The thing is the calls stopping coincided with him falling. The garbage/gaster noise after he apparently left his apartment in a hurry the day he was going to be evicted with the phone still on the line. Certain things don't line up if it was just all gaster. Why would gaster spend YEARS helping a funny email salesman guy get successful and hook him up with advertisement gigs? He's a mad scientist, not a businessman. Mike being supposedly associated with TV business of some kind lines up with the whole ads and sales type of stuff. I'm pretty sure Gaster revealed some generally forbidden knowledge type shit to spamton, like the nature of the world, the location of the abandoned mettaton robot body, and how to transfer yourself/merge with it to gain more power (freedom). The "I WON'T HAVE TO BE ANYBODY'S PUPPET ANYMORE!" line from before he notices the fucking vines the body's tangled and dangling from isn't a literal puppet thing and 100% IS a thing about him being used by people he trusted in the past. Spamton is fucking suffering from the moment you fucking meet him and hides it behind his [[[wacky sales guy!]] persona, albeit extremely poorly.
One thing that stuck out to me about spamton is how in a way he's kinda a combo of elements of burgerpants and all 3 of the ghost cousins from underale in a way (Napstablook/Mad dummy/Mettton).
Burger:
Barely hidden despair boiling under the surface. Funny/depressing shop with theme that's just a slower version of another recurring song. Hell vacations.
Napstablook:Style, lives in the shit rot, funny depression (again) He's got the least in common really. oh no...
Mad dummy: visible anger issues and erratic behavior, lust for power above all else but not for a malicious use of it. Optional rematch path where you fight him in a gimmicky stolen body of some other thing
MTT: WANTS TO BE (AND WAS) A BIG SHOT! Sells a shitload of nonsense products, ended up losing touch with the people he used to know after he got big. has the power of NEO (but only if you do the optional quest that unlocks the fight) NEO Body is also literally a scrapped Mettaton prototype body.
Rambled about 2 fucking things involving the same guy in one go, now I'm gonna go die and revive inexplicably a few hours later.