How to forward a call to another number if it's busy?

Question

Telephony

Tell me, please, the means by which you can implement the forwarding of an incoming call to the city line in case the number is busy?
There is a second landline number and a mobile phone available.

Answer 1

This is done in close cooperation with your mobile operator.
For example, here is the instruction for MTS .
Other operators google for words: bilafon call forwarding

Answer 2

Which number is busy? Domestic or urban? Single channel number?
If you are interested in forwarding calls in the event of a busy city number, then this should be done by the operator.