Problem with inbound calls

I have this line in my extensions.conf:
inbound, n, Dial(SIP/Phone1&SIP/Phone2&SIP/Phone3);
Everything works fine, but if someone calls me when phone1 is busy there is only phone2 and phone3 ringing, even after phone1 hangs up. Do you have any ideas how to fix this?

If your phones have atleast 2 lines (for incomming VoIP calls), turn on call waiting on the phones.

And how would I do that?
Somehow I can’t find anything about this on google.

The choice of lines to ring is made at the start. The busy phone will not be added back in. I believe this is intentional, so there is nothing to fix.

Multiple lines on one phone is a fairly standard feature, which should be described in the phone’s user guide. You would have to use lines with different numbers, rather than the same number, as, for the purpose of the & notation in Dial, only one call will be attempted to each device address.