Restitution is not a "fee." It is the amount needed to compenstate the victim of your crime.
The court cannot arbitrarily reduce restitution. If the evidence shows you stole $55,000 from your employer, then the judge must order you to pay that amount.
If you do not pay the full amount of restitution while you are on probation, the victim doesn't even have to sue you. The restitution order will be converted to a civil judgment, which is the same thing your employer would get if they sued you and won. They can use that judgment to attach your property or garnish your wages until the full amount is paid.
There is really no way to "dismss your record" in California. You can apply to have the conviction dismissed if you successfully complete probation -- but failure to pay the restitution would allow the judge to deny your motion for a dismissal. Even if you got the case dismissed under Penal Code 1203.4 (which is sometimes called an "expungement," even though it doesn't really expunge anything), you would still have to disclose the conviction on any government application for employment or a license.
It sounds like your offenses are all "wobblers" that could be reduced to misdemeanors... but a judge would probably not grant a motion to reduce them unless restitution is paid in full.
Phew! That's a mouthful! It seems to me that you were ordered to pay restitution in the amount of $55,000 rather than an restutution fee. That is, that was the amount of the loss to AT&T, as opposed to a fine.
You do have a right to have a hearing to determine the amount of restitution owed. This is usually done at the time of sentencing. If you have already had that restitution hearing, or if you agreed to the amount of loss and it became the order of the court, your remedies are very few if any. If the court reserved that issue you have the right to go back and litigate it.
The court can place you on probation for up to 5 years. But the court cannot violate your probation because you didn't pay restitution, if you do not have the ability to pay it.
I have no idea whether or not AT&T will sue you. That depends on many things. You will just have to see what happens at that time.
You can make a motion to have your record "expunged," as we used to say (now called Penal Code section 1203.4 relief), after your probation is completed. But if you still owe restitution at that time, your chances of having your motion granted are also slim.
You need to consult with a qualified criminal defense atorney to review all the facts and findings of the court to give you the best advise. He/she can look at all the specifics and give you proper direction.
Your probation can be extended for failure to complete restitution payments. On the other hand, you cannot be incarcerated for debt. So if you really make an effort to pay and you cannot, you should not be sent to prisoned because that is imprisonment for debt.
Talk to your lawyer.