I assume this is for a remote Teleworker phone? If so, it is most likely an Internet or far end issue... If it continues, standard troubleshooting practices apply and use TNA (Teleworker Network Analyzer) from the far end to see if there is bad jitter or delay. Then again, if the user doesn't complain followup with them and if all is good just let it go. I have seen this before sometimes and talk to the user and they have no issues... No point in fixing a problem that isn't one.
This can occasionally be a bad phone, if everything looks fine, try swapping it.