Are there any standards which consider safe destroying of VOIP phones? I would like to prevent eventual data leakage after devices which have left the company.
Considering every VOIP phone is different, it's kind of hard to standardize depending on where the storage module is located.
There are companies who are specialized in hardware destruction, opting for performing destruction yourself can be very very tricky. Should you be able to locate the storage module you could use the Guidelines for Media Sanitization produced by NIST.
NIST has created SP 800-164, "Guidelines on Hardware-Rooted Security in Mobile Devices". It is a document that discusses not only the destruction of the device, but other security features that you should be considering, including remote wipe, certificate management, device integrity, using isolation to address various threats, key storage, BYOD, etc.
There is a lot to think about when your organization begins to extend its footprint into the mobile arena, and sanitization is only one aspect.