Using The Flash Function - Nokia 252C Owner's Manual

Using the Flash Function

Your cellular system may offer a flash function service.
This system service lets you accept or make a second call
when you already have a call in progress (e.g. set up a
conference call), or send other information to the network,
such as credit card numbers.
Pressing SEND sends a displayed number to the network
as a flash, unless it is preceded by a 'p' or 'w'. (In these
cases it is sent as a Touch Tone.)
After the phone number has been entered, you can add
extra information which is for 'flash' sending to the network.
Contact your service provider for further details.
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