Actually a good question by the OP.
Meter/Temperature probe (if present):
The meter can be calibrated by simulating a mV signal to the meter to assure the meter does not accumulate error (which by my experience, they rarely do). If the meter starts to drift and accumulate error, it may need to be adjusted/replaced. Rarely does that occur, but it can happen.
Some pH meters have external temperature compensation probes. If your meter has one, then it should be verified for accuracy. If your samples are always at the same temperature as the buffer solution (room temperature) when you measure pH, the temperature compensation may not be needed. If this is the case, then I would disable external temperature compensation and remove the temperature probe. But do assure that
all pH measurements with that meter are at the same temperature. Prior to disposing/wanting to adjust a meter, assure the pH probe is cleaned/ replaced/ and the buffer solutions are fresh. 99%+ of issues will be with the probe or solutions.
pH Probe:
Probes are quite a bit more sensitive and much more variable. How often verification is made on the probe will depend on how often it is used.
If it is used more than once a day, I would buffer the meter in the morning and again after lunch (again... if the meter is being used all day). In other words, I would suggest having a four hour "hold time" for a verified meter. Otherwise, verify the meter prior to use.
Procedure:
If you don't have a procedure for pH meters, I
strongly recommend you write one, and assure that people train on it. If you don't have one, the mfg. of the meter you have can probably provide you some assistance. I have attached a sample one I got off a search that can give you a start.
The fluid in the probes need to be fresh, and most importantly, your buffer solutions need to be fresh. It's way better to order smaller sizes more frequently. I know... kind of a pain. But you will have pH measurement problems if you don't have fresh buffer solutions.
I would proceduralize who orders the buffer solutions, and who collects (and disposes) of old buffer solution.
Training:
I know we all throw training around a lot. But it's pretty important to get everyone working with pH meters around and go through things together. There are a lot of little bitty things (including probe storage, "scraping" gunk off a probe, etc.) that everyone needs to be on the same page. Otherwise, you will start to have measurement issues.
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Ok... I blathered long enough.

I hope something here helps.
To your question, the need for an accredited service on this meter depends on your contractual/ regulatory requirements. I would think having an adequate procedure, people trained on it, and performing (and documenting) routine verifications with traceable solutions prior to use should be sufficient. If you are purchasing your buffer solutions from a reputable provider, they can provide Traceability documents for the lot of solutions. Be sure and document the slope where you know when the probe probably needs to be replaced.