Personally I would regard this initially as a managerial/HR issue to begin with rather than wade in with H&S related issues until required to.
If the employee has a hearing problem there should be a record of this or brought up in manager/employee meetings (if that happens?)

Jay - Regulations about noise exposure are most unlikely to apply in this case which, in any event - as MB1 suggests, is more of a HR employee relations matter than one for OS&H.

As a hearing impaired person myself who relies on hearing aids for both ears I sometimes find it difficult to get my speech volume right when using a phone, especially if the person at the other end seems quiet, perhaps because they have a quiet voice or the phone doesn't have volume adjustment. It can also be problematic if there is significant background noise which tends to prompt me to speak louder.

One way to deal with the situation could be for the affected employee to have a tactful word directly with her colleague. However, this may depend on how well they know each other and get on as colleagues. Perhaps they don't - and possibly the situation is even being used as an excuse by the affected person to get to work elsewhere. The hearing impaired employee may well know from non-work situations that she sometimes speaks too loud, and perhaps won't be upset by what her colleague says. In my case, for example, my missus doesn't hesitate to say "no need to shout" or similar - so I'm used to such advice, don't take offence and can respond accordingly. However, if the affected employee is wary about mentioning the matter to her colleague, another approach may be for someone else like a supervisor or a friend/another trusted colleague of the hearing impaired employee to visit the room and comment accordingly with appropriate tact and humour.

Another possible factor is that the phone used by the hearing impaired employee might not be suitable or needs adaptation. Until several years ago, my colleagues and I used analogue phones with handsets which each incorporated a hearing induction coil. Simply switching one or both of my hearing aids to "T" setting to pick up the signal from the induction coil enabled me to use such phones with relatively ease. When my employer changed to digital type desk phones, I was dismayed to find that the new ones had no induction coils. Also, it seemed that the designer/maker hadn't even considered offering handsets with induction coils as an option. However, my employer responded very reasonably by obtaining a 2 earpiece headset with microphone which I can plug into the phone and works moderately well along with a device which enables me to adjust the volume of the incoming sound. The device also allows some adjustment of the sound tone which is important because clarity of speech is as important as volume. However, though this arrangement worked for in my case (though it's not as convenient as having handsets with induction coils), it might not do so for the hearing impaired colleague you describe.

Another thought. I realise that I've made an assumption so far that the hearing impaired colleague knows that she is partially deaf. Perhaps she doesn't realise or doesn't want to acknowledge the fact - even if friends and relatives tell her. For many people with impaired hearing the onset of impairment is gradual, so they don't realise how much their hearing has deteriorated. Therefore, as a piece of general advice, such people should be persuaded to ask their GPs to refer them to a hospital audiology department. If this leads to them getting appropriate hearing aids, the aids can make a big improvement for them in social as well as work situations.

I can only suggest that this simply isn't your problem. The "Ops Manager" is tasked with managing this.
If you were approached by the individual in preference a direct approach to the Ops Manager there may be several reasons, sensitivity being the most obvious.
The response you describe ( the Ops Manager unable to physically locate anyone) suggests an unwillingness to engage with employees first-hand to discuss the matter. Nevertheless, this is the job of the manager. If he/she needs support with that then they should speak with HR professionals in your Organisation.

Long story short: Managers must........manage.

Thanks for all your advice, I had initally passed it straight on to the HR manager as I agree it is more of a personnel issue. I just wanted to get feedback in case it was thrown back to me regarding noise levels and such. I think the comments about adapted telephones is a good one to loook at.

Yeah, I not too convinced that this is a H&S problem as such. It needs to be approached with sensitivity, perhaps the person doesn't reaalise. You might find help from the british Deaf Association.