The use of cement mortar as filler on historic timber-frames is commonly seen but it is wrong and destructive. It has been in common use in the last 60 years or so by the general building industry, probably without realising the damage it can cause. It is a short-term fix but leads to longer-term decay of the frame, moisture absorption and it can create a habitat for beetles. It also looks unsightly, especially when bits of the cement mortar fall out.
There is no option for repair other than carefully removing it, to allow for an inspection of what is hidden so that a decision can then be made about what to do.
It might be possible to do a specialist oak-frame repair or perhaps to use an appropriate lime filler instead. Seek advice from a conservationist before doing anything.
If the damage is relatively minimal and oak frame repairs can be avoided then it should usually be fairly straightforward to do a repair using a well-haired lime mortar. This should be packed into any deep holes. The hair helps to reduce shrinkage of the lime. The abutment of the frame and the brick can then be pointed with a burnt sand mastic. This is a mix of burnt sand and linseed oil. It is very sticky when applied but slowly hardens over time to provide a flexible joint. It is a traditional technique that has been used for hundreds of years and is still used in conservation today throughout the world.
If the damage is more severe then you will need to talk to a specialist oak framer about what options there are to reconstruct the joint.