Someone at my home was served a 'summons' addressed to me two days ago. In the upper left corner is detailed info for plaintiff's attorney. Centered toward middle of page it says 'in the san marcos justice court, county of maricopa, state of arizona' beneath that it lists plaintiff vs. myself and jane doe. Under that is states: this is an official document from the court that affects your rights. Read this carefully. If you do not understand it, contact a lawyer for help. (end) it then states: the state of Arizona to the above named defendant(s) and again has my name and jane doe (along with my current address) The second page reads: "a lawsuit has been filed against you. a copy of the lawsuit and other papers are served on you with this 'summons' YOU ARE HEREBY SUMMONED AND required to appear and defend this action in this court. you shall appear and defend within 20 days after service of summons and complaint, exclusive of the day of service. YOU ARE HEREBY NOTIFIED that in case of your failure to appear and defend within time applicable judgement by default may be rendered against you. in order to appear and defend you must file an answer or proper response in writing with the clerk of this court accompanied by the necessary filing fee...." it is signed and sealed date is over 20 days ago it is accompanied by affidavit of indebtedness and is signed by attorney for plaintiff with the typed month crossed out and previous month to that hand written in place. Does not list any other contact info except for attorney for plaintiff. Case # begins with 'cc' which I can find no others beginning with 'cc' all others begin with 'cv' in case # lookup. Is this real or a longshot from an overzealous attorney?

This sounds real. You can call the court clerk where it was filed to confirm.

Without looking at it, and comparing it against summons issues by your court it is hard to tell. You call or go down to your local court and check to see if that is theirs,

Sounds like you are being sued and this is a sumoms to appear in court

On a bill that is 10+ years old in which the last payment was made 5 years ago, can it be collected?

I received a summons today. I called the law firm. They offered a settlement to be paid in 2 payments of over $2,000 each. I said I could not pay that much but would pay $100 a month. They won't accept that.

Joe - call the Clerk's Office and ask them if they have a case of THEM vs. YOU and see what they say. Also find out just when your Answer is due in the Clerk's office to make sure they don't get a Default Judgement against you.

It sounds real to me. Probably sent by a junk debt buyer that has no proof or records of your debt beyond that affadavit. One collection agency, "lawfirm" in Atlanta is purported to file 127 "debt collection lawsuits" per day!
These guys are a mess in some cases and are cranking out these lawsuits hoping you'll not respond (I think over 90% of these kind of cases end in default judgement because the Defendant does not respond) and they'll be granted a judgement so they can go after your wages and assets.
First off, do check with the court listed on the summons and make sure it's real. That can be accomplished over the phone or even online depending on your State.
If it is real, file a Notice of Appearance, Answer, and Certificate of Service. Once you've responded, they'll likely just drop your case and move on the next unsuspecting one...
You can find templates for those forms at E-Document Package
Good luck!