you're being too hard on yourself.
working on the phones is tough for most people during stable times, let alone after you've been ill or had a depressive episode.
You can go back if you wish, i'm sure they will understand, they sound to be good about that stuff, but i would seriously re evaluate why (other than financial, as big as that reason is) you want to stay there? is it beneficial to your health and well being? only you can answer that.
((hugs))