Evan,
I'm with Navy/Marine Corps MARS, but the basics are the same in all of the services. The requirements for joining MARS are as follows:
1. Must be 18 years of age or older
2. Must be a US Citizen or resident alien.
3. Must possess a valid amateur radio license issued by the FCC or other competent US authority.
4. Must possess a station capable of operating on MARS HF frequencies.
There are also minimum participation requirements, which are easy to meet.
I can't address your question regarding running a phone patch using DSN lines, but an important point you need to know is that the passing of morale and welfare traffic is a rapidly-diminishing mission of MARS. The primary mission of MARS is now directed more toward the provision of Department of Defense sponsored emergency communications on a local, national, and international basis as an adjunct to normal communications. In that respect, we are becoming more and more involved in homeland security, and have frequent training and ECOM drills to prepare for that mission.
Your final question is regarding callsigns. MARS members can have multiple callsigns based on the positions they hold. For instance, my basic MARS callsign is NNN0TPR. I use that callsign to identify my station on our daily nets. I also serve as the Northeast Area Assistant for Public Affairs/Newsletter, for which I am assigned the administrative callsign, NNN0ASI EIGHT. That callsign is assigned to anyone who is in that particular position. I sometimes use that callsign to identify myself on some area nets, or in correspondence. It's a similar arrangement to the way the army, for instance, refers to it's staff functions (eg., G1-Personnel, G2-Intel, G3-Operations, etc.).
Hope this has helped. If you have further questions, you can visit our website at
http://www.navymars.org (links to the other services websites are there also), or you can e-mail me at nnn0asi8@navymars.org.
73
Dwight NNN0TPR/NNN0ASI EIGHT