If your memory battery has not been replaced.....

The 3.6v ni-cd battery inside your phone system IS leaking battery acid and will eventually kill your phone system. It can take years but that time could easily be up now.

Ni-Cad batteries have a life of about 5 years. Most of these systems are 10-20 years old and many still have the original battery inside. 95% of the systems we see are slowly leaking battery acid and most have large corroded areas around the battery.

System defaulting? Phones not ringing?? When the battery goes bad, the system will loose its programming in power outages. At default only phone #4 will ring. However, even if your battery has enough juice to keep programming in short power outages it is still leaking acid and will die completely any day now.

What can you do about it?
The battery is soldered in and is not easy to remove or replace. If you (or someone you know) are comfortable taking electronics apart and willing to give it a try (at your own risk!!) you should at least cut out the old battery out to prevent damage from battery leakage. Either solder in a replacement (any 3.6v Ni-Cad or Ni-MH will do but it requires some soldering. You can buy a cordless phone battery with wire leads and solder the red to the positive side and black to the negative) OR the easier option is to put the system on a UPS battery backup system made for a computer. UPS systems are available at office supply stores, Wal-Mart, Best Buy etc.. Your phone system uses a lot less power than a computer so any computer UPS will do just fine and last longer than if used with a computer. Of course, a UPS power supply will not keep the programming in a long power outage like the internal battery would.

MT-16H (and Prostar 816) Control unit battery location:

ProStar Manual
Macrotel Manuals

Macrotel Excel control unit battery location: