The intercom supports mobile phone numbers, landline numbers and international numbers. You can have up to 8 numbers connected, they will be rung in sequence meaning the 1st number will be called and if it fails it will divert to the next number after 10-30 seconds depending on the system configuration*. 

*If a voicemail is present on the number that the intercom is dialling it might be worth increasing your voicemail pick up time to allow the intercom to ring you for the full ring time before it diverts. If the intercom reaches a voicemail system before diverting it will not continue to call the other numbers as this will count as the call has been answered.