A 2-yr probation period is unheard of.
Additionally all employees have an entitlement of sickness absence accounted for.
I do not believe a two-week sickness absence in Oct 2015 and a further 2-week sickness absence in March 2016 can lead to a dismissal.
They can and may discipline you for absence periods.
You say you had no contact from your employer?
Did you not contact them advising them you would be off sick? Employers have a duty of care so if you are sick they can insist you attend an independent doctor for assessment.
No biggie if your genuinely off sick and is normal practice from employers.
You should only be contacted in a formal professional manner.
Its unusual that you have been unable to contact the attendance manager.
Will your employer believe you tried on four occasions but could not speak to anyone.
You are responsible for keeping your employer up to date, failure to do so can result in dismissal.
You do not need to attend the workplace for a site visit,
you can have staff visit you at home or in a close location.
Its a hope you are well meeting.
Its unlikely they would withhold a bonus payment but if from the employers perspective it appears you have made no effort in keeping them informed of your health issues they may be able to give you problems, warnings etc.
I do not think they would have the right grounds to terminate your employment.
Good luck. If I was you I would be at the workplace Monday morning demanding to see the attendance manager to resolve and put your mind at rest. But that's just me buddy