I don't work for skywest, but many times I hear skywest flights on the freq. with alphanumeric callsigns. At mesaba the only time we'll have a 3XXX-A flight number is when another aircraft is airborne using the same 3XXX callsign. (e.g. flight 3630 may operate RAP-MSP-DFW. If RAP-MSP is running late and MSP-DFW gets off to DFW before the RAP inbound gets to MSP, it will be MES3630A as far as ATC is concerned.)

I would imagine at skywest...operating hundreds of flights a day for both DL and UA in the same region (out west) this probably occurs on a daily basis.