I can't think of any legitimate reason they would subpoena your employer to attend your Chapter 13 hearing, but sending your employer a subpoena probably isn't a violation of the automatic stay per se. However, when they called your employer post-petition to discuss your work (presumably in connection with their state court lawsuit), that likely IS a violation of the automatic stay.
Ultimately, the Trustee will have a lot of power in this scenario. You and your attorney should have a sit-down meeting with the Trustee and determine whose side the Trustee is on (if the underlying claim is meritless, the Trustee could well be on your side), whether the case is likely to proceed in bankruptcy court (rather than state court), and whether you will need hire a civil litigation attorney.
If you're going to be litigating in bankruptcy court, there's no way to sugarcoat it - it's going to be very expensive for everyone, unfortunately.
One important question, though - Did these alleged creditors ever file a Proof of Claim in your bankruptcy case?
Also, what are the creditors suing you for, very briefly? A little background might help.