Can you call other numbers?
If so, what is different about your mother's number, and can you phone other numbers of the same type/carrier/area code? If you can narrow the scope of the problem and identify what type of number it applies to that will help identify the cause.
Have you tried the other "generic" fixes: reboot phone, wipe the phone app's cache, boot into recovery and clear the cache partition (if the phone has one)?
Was this a major version update or just a security patch (or a bit of carrier bloatware being added)? The ultimate resort if the problem was caused by an update (we don't know yet, correlation is not causation) is to back up your data and factory reset the phone. Over the air updates that don't erase anything are by definition "dirty", and there's always a small risk that some data on your phone somewhere will be incompatible with the new software, so a reset is always worth a try when things misbehave after an update. However it's also a pain, so I'd always try to identify the root cause first if possible.
Final thought: have you spoken to your service provider, just to check that there's nothing wrong with their systems?
(That's all I can think of at the moment).
