That depends on how you allow/restrict traffic between the sites. If you don't force any specific rules for all other inter-site traffic, then don't need be any more specific here, as well.
That alone will guarantee inter-site communication. Now, if your PSTN traffic exits, again, through the Internet (eg through a SIP trunk), you will have to adjust accordingly your edge firewall. If you use ISDN then no additional care needs be taken.
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