provide them the warranty service if warranteed issue, training, support, normal back ups as, and if you contracted to do.
for what your contract does not specifically cover, bill them time and materials, with standard refusal of additional service if their account is far enough in the rears. you can stipulate in a contract that after so many days in the rears you are no longer liable to provide support including warranty support for issues occurring after an original sign off on completion of the project.
if you contracted to train them to administer their own system then do so. it is common to include some basic system admn training, but this varies by the installer. i usually cover time/date, ringing, adding vm box, etc. i give them the manuals, and quick reference guides for all their components. during training if they are prompt, attentive, and do not walk away during the training time, i will go a bit more in depth as time permits compared to time allocated for training[about 30 minutes for admin]. if they need further training after contractual training is complete, that is billable.
i get a sign off on the installation stipulating completion of install, user/admin/vm training, at which point the free work is done. if they want more i let them know this is billable, but may give them a little more help at no charge after explaining i will not bill them just this once as a curtousy, but additional service will be billable. we in general include one return trip for changes within the first 30 days at no charge, and advise the customer to collect desired changes for one visit to avoid additional charges.
i do mark the pages for programming access, and write their passwords in the manuals where the manual covers accessing the systems. i note on the flow chart page in vm manual their night aa, day aa, other aa's. and the number sequences to reach them.
if they need further help, i will schedule a call asking them to have their manuals i marked on hand for the call. then walk them through the manual which i noted thier paswords, aa's, etc. on during training. if this is to much for them, or lasts more than about 10 minutes i inform them i am going to need to schedule a service call to come out. write a wo with the scope of their needs, and email or fax it to them with a proposed date, and time for your visit. make sure the wo shows the work is billable, and the rate for the service.