Many P25 systems are able to connect to other P25 systems via an ISSI connection which allows subscribers to interoperable between systems.
All P25 systems are capable of ISSI linking; it's part of the APCO 25 standards.
The only thing is that both systems would have to be Phase II.
False. ISSI works with FDMA, TDMA, and mixed FDMA/TDMA systems. The whole purpose of ISSI is to facilitate the linking of systems, regardless of the manufacturer of the infrastructure, the brands of radios being used, or the modulation used on either system.
Also, many systems have conventional simulcast channels for some of their talkgroups so that mutual aid resources who are not on their system are still able to communicate with those operating on that talkgroup.
That's a real stretch to say that "many" systems have links between trunked talkgroups and conventional resources strictly for the purposes of interop or mutual aid. If you took a formal survey of how many P25 systems have the ability to deploy those sort of links, I think you'd find the number to be surprisingly low.
Interoperability should be a mindset more than something you throw technology at. There's nothing wrong with having a P25 system for your city or county, which in many respects can and does provide interoperability between agencies using those systems on a day to day basis. Those sorts of systems can bring agencies together who were previously using disparate systems or bands, and so they have value in that respect. However when it comes to large scale incidents where agencies outside of the normal operation of that P25 system are involved, the best practice for interoperability is to have a good plan in place beforehand. That's first and foremost, as you don't want to have to figure it out in the heat of the moment. It's also best to fall back to the lowest common denominator, which in many cases can be the use of simplex conventional channels such as the nationwide CALL and TAC frequencies.
Save the big fancy trunked systems for day to day ops, and have a plan in place with your surrounding neighbors, state, and federal agencies as to what common channels are going to be used when the $#@% hits the fan.