Call centers are the
devil when it comes to weight gain! I put on sixty pounds in a year working at a hotel reservations center here; it's so easy to do when you are basically chained to a desk for eight hours a day.
You're right, 800 calories a day is untenable (and probably quite unhealthy, for that matter). It's also going to be really tough for someone with a small amount of weight to lose to manage two pounds a month; in fact, the usual recommendation for safe fat loss without undue muscle loss is 1% of your body mass--so for you, that'd be a maximum of one and a half pounds. More than that and you risk losing stuff you want to keep.
If you can actually ride a bike and take calls, you are already in phenomenal shape.
It's tragic how out of breath I get from riding mine (which reminds me, I have yet to get on the hamster wheel today). You probably can stroll around in your little cubicle or veal-pen or whatever you're set up with at your center. Even taking calls standing up will burn more calories than taking them while sitting.
Use your breaks, assuming you get a couple of fifteen-minute ones, to go for walks or up and down some stairs, if your building has them. Every bit helps offset the sedentary nature of your job.
Good luck with both the weight loss and the job!