Is IP telephony from two sites on one PBX possible?

Question

Telephony

Good afternoon.
Who will tell such a moment.
There is an automatic telephone exchange Avaya with carrying out on 20 numbers. Located on the same site.
There is another site, which is connected to the first Rostelecom channel. There is N number of Avaya ip phones.
Question from the teapot in this. Is it possible to somehow connect these phones to this PBX?