As this dictionary puts it I hear what you are saying means to acknowledge what someone has said. But in the 13th episode of the 7th season of Friends Phoebe said I am hearing what you are saying. Here is the context:
Phoebe: (on phone) Hi, this Phoebe from Empire Office Supplies, can I speak to your supply manager please? (Listens) Earl, thanks. (Listens) Hi Earl, this is Phoebe from Empire Office Supplies I’d like to talk to you about your toner needs. (She’s reading from the script.)
[Cut to Earl’s office, who is played by Jason Alexander, George from Seinfeld. They cut back and forth between Phoebe’s and Earl’s offices with each of their lines.]
Earl: I don’t need any toner.
Phoebe: I’m hearing what you’re saying, but at our prices everyone needs toner.
Is there any difference in meaning between I hear what you are saying and I am hearing what you are saying?