Two numbers ringing at once.

[chuc3]

I have two numbers in different locations and i need them to ring at the same time when either number is called .
How can i achieve this ?

[tot3nkopf]

Many ways. Some options:
-tandem sets
-set supervision

[chuc3]

I can create a mobile to ring at the same time as a analogue however I'm struggling to create a analogue "A" to ring when analogue "b" is dialled and vis versa.