I would suggest using a PRI. Wanye67 is correct that QSIG would be the easiest. But you could use almost any protocol. NI, NI2, 4-ess, 5-ess, etc. You would just have to setup one side to look like the central office (aka CO) switch. If you decide to do this, let me save you some time. It will take days for the TAC to give you this command.
isdn protocol-emulate network
Apply it to the serial interface of the PRI and it will turn the Cisco router into the central office switch (kind of, at least to the other device).
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)