I do not understand why you are coming on here for advice, when you state you have a union rep.
I am a case officer for a banking union, and we are best placed to give advice. We know the employer, we know the offence. and we know the likely outcomes.
Call dropping or avoidance is a Gross misconduct offence, and suspension is the normal route. At your investigatory meeting you will be asked for your side of events, and any evidence will be put to you. Notes of the meeting will also be taken.
After the meeting you will receive a written invitation to a disciplinary hearing (unless they decide there is no case to answer) with copies of all documentation that will be used during the hearing.
At this point you will need to discuss with your union as to how to proceed.
It may be, if the case is serious, that resignation would be the best option, if the Business would allow you to leave with a clean reference. If not. then it may be better go through with the hearing.
If you are dismissed you have a right of appeal, although appeals are hard to win.
A reputable Business is unlikely to take the fact you are pregnant, or approaching two years employment into account when carrying out the disciplinary, however, I could imagine that a 'less scrupulous' employer may do.
You pay your union subs.......make use of them!!!