question

Yes.  Powerline adapters are flaky at best and should be considered a last resort for devices that need reliable connectivity like a phone.  They can be fine in a home for a computer that does web browsing, etc. but personally, I would never use one in a business environment.  Just get a cable pulled where you need it.

Ideally, pull a cable.

That said, I have connected RC phones to a Linksys PLSK400 pair and also the TPlink AV1200 pair. The linksys is by far my favorite, as I could have the desk-phones, workstations, printers all connected to the network via the adapter. Never any problems. Eventually, we pulled cable anyway and stored the adapters in case they were ever needed again.

If cable isn't practical for you for whatever reason, swap some phones around, see if the problem follows a particular phone or stays in one place. Make sure all your cables are good, or try a different wall plug for that phone. 

That's all I got, good luck!