situation - Bank Teller and his mother

I found this on 9GAG, albeit without an answer. I am really curious on what it is. It's copied word for word from 9GAG, hence the terrible english.

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You work at a bank and suddenly a gang of robbers shows up. One of them comes at you and tells you to give him all the money or he will shoot you. You tell the robber you don't have access to the deposit.

Suddenly the phone rings. The robbber tells you to answer the phone and don't say a word about this. You pick up the phone and it happens to be your mother.

After you talked to her, 15 minutes passed and the police arrived along with your mother and arrest the robbers.

What did you tell your mother at phone such that she understood you need help and the robbers couldn't notice?

Hints:

• The phone you used was a ring phone (not cell phone);


• You used a "little trick" when talking to phone with your mother;


• You didn't instruct your mother for this emergency, so she didn't know about a secret code for help.

Answer

The trick is:

You ask your mother questions like "Is this an emergency? I'm busy at work" and say "I can come round later and help." but you use the mute button so all your mum hears is "emergency... work... help" etc. The actual words don't matter as long as you can filter in the message.