On call means that during a period between two tours of duty you are required to be contactable and deployable to recall to duty immediately. There is an expectation that you are physically fit and able to be recalled at short notice. This differs from the general recall to duty that all officers can be subject to.
The on-call allowance was introduced following the Winsor recommendations and came in force from 1st April 2013 and is not retrospective. All federated ranks are covered by this allowance.
An officer "shall receive an allowance of £15 in respect of each day on which he spends any time on-call" - para 13) Annex U to determination for Reg 34.
It should be remembered that on-call is entirely voluntary and is desiged to be used where it is not practicable or reasonable to have it covered by a 24/7 shift pattern.
The Force currently has a list of recognised on-call posts.
If you feel that you perform on-call but do not receive the allowance please contact your local Reps