Normally, a lawyer must advise a client in writing of the basis upon which the client will be billed for services.
By far the most common way to do so is on an hourly basis with a fixed hourly rate for each professional in the firm, with time customarily kept to the nearest tenth of an hour.
Sometimes lawyers will instead bill on a flat fee basis for a particular task and include that in a fee agreement. This would have a set of services and the flat fee for each and might say that other services are billed on an hourly basis.
Still other lawyers will bill for their services on a contingent basis.
Sometimes, in practice, a lawyer will have some routine standard time increments that are billed for certain brief tasks, whether or not this is expressly spelled out in the fee agreement.
For example, most lawyers will charge a minimum of 0.1 hours for any kind of billable work (e.g. a brief e-mail or review of a court document that was received and instructions to staff to calendar any associated deadlines with the filing received), regardless of the actual duration, and many will charge 0.2 hours or 0.25 hours as a matter of course for all but the shortest of telephone conversations to reflect time spend preparing for and following up from the call and time spent transitioning from one task to another, and to avoid the necessity of putting every call on a stopwatch. Calls longer than 12 to 15 minutes are then billed based on actual length instead.
As a rule of thumb, 0.1 hours per short email in an email chain, and 0.2 hours per short telephone conversation of more than a few words, would be pretty normal, with longer amounts billed for a lengthy email or a long telephone conversation.
It also wouldn't be unheard of for a lawyer to have a set amount of time billed for certain common, short tasks, like drafting a motion for extension of time and getting it filed, or to have a flat fee for certain transactional drafting fees like drafting a simple will or a deed, or to have a flat fee for an initial consultation.